Not completely sure if this is a bug, but I bet it is.
Since illusions are a prohibited school to enchanter, trickster/enchanters can't use trickster spells, they are just greyed out. I assume this also applies to Conjurers but didn't test it.
Funnily enough, trickster/illusionist on the other hand doesn't get increased power with the trickster spells.
So in one case they count as wizard illusion spells, and in the other it doesn't.