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Consider the following two assumptions:

1. 
   Validity Assumption: Assume an argument is valid. It follows all the formal logical rules of inference. The inference contains no formal logical fallacy.


2. 
   Soundness Assumption: Assume the premises of the argument are sound, verified by a competent subject-matter expert.

Given the soundness assumption, the validity assumption would imply that the conclusion is logically true.

Is it possible for this argument to still be an example of an informal fallacy?

What makes me think this is possible is that establishing the soundness assumption, which I assumed to be true, cannot be done with absolute certainty. The subject-matter expert verifying the premises as true may have made an error of judgment. The validity assumption is more reliable as an assumption than the assumption of soundness since it can be checked with a computer without involving human judgment.

This would make the list of informal logical fallacies valuable. They would be ways to test sound and valid arguments by identifying places where the argument could go wrong.

What I am looking for are examples of such situations that would answer the question in the title as "yes" or an argument that such examples are not possible.

To repeat the question: Can an argument be formally valid with sound premises and still be informally fallacious?

I say yes.

Consider two people who don't know the color of bananas, and are trying to figure it out through an argument. One of them provides the following argument:

Bananas are yellow
Therefore, bananas are yellow

It's clearly valid, and any subject-matter expert would agree with the premise. But, the second person will (rightly!) object that this argument commits the fallacy of circular reasoning/begging the question

This question is only meaningful if one ask about the status of the so-called informal fallacy, as a philosophic concept. The current academic usage differs from the medieval usage. All definitions are circular in the sense that something is said about something, for example, Kant says in the extreme case the subject and the predicate are the same: "a book is a book." If I say a book is a "readable thing," one can still object that the readable thing could as easily be the subject that supports the claim or predicate; this readable thing is a book. The objection is wholly arbitrary in the current academic usage. And in fact it has a formal character, though it is called informal since it is not a matter of an inference as are the "formal" fallacies.

The answer above "Bram28" is wrong by the medieval standard. It's not circular (in the negative sense of being a fault) if the premise is accepted by the intelligence of the one involved in the disputation or the dialogic investigation. Circular in this context is another name for not sharing the assumption of the other about the premise. If I am color blind I may not believe you when you say the banana is yellow. I don't share your premise. Than I say, this is a circular argument, and what I mean is you have not given me anything to move or persuade me to believe your view that the banana is yellow. I need something more. What you say is intelligible, but it does not truly move me to where you are, objectively in my knowing. You don't demonstrate to me, but you instead propound in a dogmatic way. I can learn what you say by rote, but am not led objectively to know in the way you do.

So these questions have to do with a different way of grasping the objective or independent character of the soul as arbiter and its internal subject matter. One might convince a blind man, genuinely and logically, rather than through appeal to interest or trust, of the color of the banana. But since I can not look, as a color blind person, at the yellow color, the mere correctness of the assertion is not enough for me: it is not a circle. It is a broken circle because one part of it is not accepted. The circle, in fact, is either vicious or virtuous on a different ground than the petitio principii, it is the ground of whether it helps me in a larger context (as with the example about the case with Justice Roberts and the "Legislature" I gave in the question about logic).

Logic and epistemology

I think you are running together logic and epistemology. If the premises are sound (true) and the conclusion is validly deduced from the premises then the conclusion is true : the argument, call it A, is valid, non-fallacious.

This is where logic's interest begins and ends. When you introduce considerations of only assuming the soundness of the premises and not being absolutely certain of their truth, this is a matter of epistemology. We often have good cause to doubt whether premises are sound but this is a matter, a problem or question of what you know about the premises. As regards premises, logic depends on whether the premises are sound, not on whether you know whether they are sound.

But there is more to say, more favourable to your suggestion.

Wider context

I see no reason why an argument, A, which is sound and valid should not feature as part of a longer argument, B, which does involve an informal fallacy. The argument, A, remains logically faultless but it might be embedded in another argument, B, which as a whole commits an informal fallacy such as Straw Man or Ad Consequentiam or any of the other informal fallacies.

In forall x: Calgary Remix the authors claim there are two ways that an argument can go wrong:

The general point is as follows. For any argument, there are two ways
that it might go wrong:

• One or more of the premises might be false.




• The conclusion might not follow from the premises.

To determine
whether or not the premises of an argument are true is often a very
important matter. However, that is normally a task best left to
experts in the field: as it might be, historians, scientists, or
whomever. In our role as logicians, we are more concerned with
arguments in general. So we are (usually) more concerned with the
second way in which arguments can go wrong.

The question I raised brings up a third way an argument can go wrong even more so than I originally realized with Bram28's answer referencing the fallacy of circular reasoning.

Wikipedia defines the fallacy of circular reasoning as follows:

Circular reasoning...is a logical fallacy in
which the reasoner begins with what they are trying to end with.
The components of a circular argument are often logically valid
because if the premises are true, the conclusion must be true.
Circular reasoning is not a formal logical fallacy but a pragmatic
defect in an argument whereby the premises are just as much in need of
proof or evidence as the conclusion, and as a consequence the argument
fails to persuade.

The third way arguments can go wrong is that they fail to "persuade", or perhaps better put, they should fail to persuade those hearing them.

With a circular argument the premise becomes the conclusion. That is a valid inference. If the premise is obviously true, the argument is sound. But, nonetheless, the argument is an example of the informal fallacy of circular reasoning and remains so in spite of the validity and soundness of the argument.

Should anyone accept such an argument? No. It should fail to persuade. More is required from an argument than soundness and validity.

Reference

P. D. Magnus, Tim Button with additions by J. Robert Loftis remixed and revised by Aaron Thomas-Bolduc, Richard Zach, forallx Calgary Remix: An Introduction to Formal Logic, Winter 2018. http://forallx.openlogicproject.org/

Wikipedia, "Circular reasoning" https://en.wikipedia.org/wiki/Circular_reasoning

Premise one: The "Bald Eagle" is a bird named for the appearance its head. (Correct)

Premise two: White Headed Vultures and Ocellated Turkeys are both birds who are bald (without feathers on their head). (Correct)

Logically Inferred Conclusion: Bald Eagles are also a bird that is bald (without feathers on its head). (Incorrect)

So, based on the true premise that there are Birds that are bald, and that the Bald Eagle is named for the appearance of its head, you can draw the incorrect conclusion that the Bald Eagle is a bird that is bald - rather than that it was named for the old and largely defunct meaning of "bald" as "white headed"

The informal logical fallacy here is a combination of an Etymological fallacy (on use of the name "Bald Eagle", and the change in the meaning of the word "bald"), a Referential fallacy (likewise) and a False Attributation fallacy (while the premise that there are exist birds that are bald is true, it hold no relevance to the true situation)