Originally, in "Vanilla" Descent , the extra wounds dealt to a creature with Undying would carry over an ignore armor. This was changed with Road to Legend , but until the latest expansion, we (our group) had thought it was limited to Road to Legend . Tomb of Ice is out now though, and it's glossary of terms label Undying without the wounds carrying over.
I could see this going either way, honestly. It may be an intentional change, as there are numerous ways to negate the Undying ability through new items and such, AND players have Feats now... or it could be a leftover from Road to Legend .
Any official word or thoughts?