Undead don't seem to have any dialogue for enhancing them any more. Tried it with both default zombie and skeleton warrior. I asked in discord and apparently this is a bug, so reporting here! I was using the animate dead sorcerer perk player tool to test. Area I was in was the ring 99 stables, not sure if that has any relevance though, but I've yet to try enhancing them elsewhere.
Minor update on this: Further tested this with shambling undead (the no corpse summon) in different areas. Still no dialogue to enhance them or any dialogue at all.
Should be fixed next reset. Let me know if still not working.