I fail to see why not, since if you have 2nd level spells from the PrC, you lose them from the feat, which means you don't qualify anymore, which means you no longer have second level spells from the class, and therefore your feat kicks back in, which means you no longer etc.
It's either an infinite loop of quantum qualification, like Dragon Disciple 10, or it works. Which one makes your game not grind to a halt? You don't lose the feat, you lose the benefit of acting like you have a second level spell from the feat.
"When you become able to cast 2nd-level spells, you lose the benefit described above but retain the extra 2nd-level spell slot,[/quote] Relevant part from the feat, italics mine.