[Esther]

Changing your subclass removes Copy Spells that were stolen from that subclass.

For example:

1.) Spellthief / Hexer, steals Quickness from a Curate.

2.) Spellthief / Hexer switches to Spellthief / Curate, to gain XP at the Dormeho clinic in order to put ranks in Quickness (so it has a usable rank as a copy spell)

3.) Spellthief / Curate switches back to Spellthief / Hexer, and the Quickness copy spell gets removed from the Skill Pool, presumably because the code that empties your skill pool of skills attached to the class you switched out of was written before Spellthief was a thing, and no~one ever thought to update it.