That is certainly one of the strangest bugs I've seen.
Were they actually Grendelkin? A pretty huge downgrade if so.
Edit: Might want to report it to the CBM team if you haven't already, although I'm not sure if such a bug can be fixed seeing as how the boosted battle forms work.
I have no problem with them. I'd rather not redo a turn as I've had bad experiences with such things, so if you can live with it, so can I, since it probably hit you much worse than me.