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Given the consensus from my previous question, it appears that all Illusions conjured from this spell 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 beating 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.