I've tried pollinating them then using falconpunch, but they end up switching back to the preg morph within minutes. any other ideas?
Yes they did try and it didn't work.
No idea how you fix it. It happened to me too but by that point, combined with about 50 other glitches, I thought fuck it and just stopped playing the game. I went back to sims 2, but that's probably not the most helpful resolution for you.