I have a 20th level wizard who has chosen Fireball and Lightning Bolt for his signature spells. According to the entry for Signature Spells, they shouldn't count against my prepared spells count:

Signature Spells

When you reach 20th level, you gain mastery over two powerful spells and can cast them with little effort. Choose two 3rd-level wizard spells in your spellbook as your signature spells. You always have these spells prepared, they don’t count against the number of spells you have prepared, and you can cast each of them once at 3rd level without expending a spell slot. When you do so, you can’t do so again until you finish a short or long rest.

If you want to cast either spell at a higher level, you must expend a spell slot as normal.

I have an intelligence of 22 with a mod of +6 so, at level 20, I should be able to prepare 26 spells + my two signature spells. However, I'm only able to prepare 26 total (and cantrips, of course).

Link to character: https://ddb.ac/characters/81021/Ppm13d

Looks like you prepared the spells normally in addition to making them signature spells.

Ah.... I removed them as signature spells, unprepared them, then re-added them and this solved the problem. Thanks!