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The one answer to my previous question suggests that all Illusions conjured by Disguise Self and Seeming can be seen through by the user, but not by outside viewers.

The two spells Disguise Self and Seeming both differ from traditional Illusion spells in that they contain no clause that causes the Illusions to "fade" or "become transparent".

Instead, successful Investigation checks allow viewers to "discern" and "become aware" that the target is disguised.

Can viewers succeeding on the check literally see through the disguise? If a Half-Elf disguises their race to look like a Drow, does the viewer know their true race if their check beats the disguise, RAW?

Spells do exactly what they say:

Disguise self reads:

To discern that you are disguised, a creature can use its action to inspect your appearance and must succeed on an Intelligence (Investigation) check against your spell save DC.

Seeming Says:

If it succeeds, it becomes aware that the target is disguised.

Both imply the success mean they know you are disguised, not necessarily what you look like under the illusion. The DM can rule what that means for your table.

Illusions become translucent only if they say so

As you mention: some illusion spells (like Minor Illusion and Programmed Illusion) state:

If a creature discerns the illusion for what it is, the illusion becomes faint

But other illusion spells (like Disguise Self, Seeming and Mirror Image) do not have this clause.

This appears to be intentional based on desired effect (especially in the case of say, Mirror Image).

Historical note (if desired) - this was also how they behaved in previous editions. In 3.5e, the "translucent outline" effect only applied to a figment or phantasm (like Minor Image or Programmed Image)

Although, as a point of interest, an investigation check is different from a perception check.

I would argue that most people would have no reason to question a disguise, you wouldn't walk down the street as a disguised drow and have everyone on the street roll investigation checks. Furthermore, investigation seems to involve being able to be up close and if need be, interact with an objects.

I would suggest the only people who would investigate such a disguise are those who have a reason to question it's validity (eg. you're wearing a fur coat in hot weather or something dislodges your costume revealing something underneath or something passes through a part of your body that isn't real etc.)

or

those who are proficient in investigation and hence are more used to questioning things/ have a natural curiosity.