I've never encountered this issue until tonight on my current play through. I was going through Halldir's Cairn to retrieve the drum there for the quest only to find that Halldir's name was replaced with simply the generic "draugr scourge" name, despite the npc having all of Halldir's abilities and so forth. I don't believe it has caused any long term issues, but it is a personal first for me. Does the game sometimes spawn unique monsters with generic names or something?

If you've beaten Halldir before, the next time you return to Halldir's Cairn he'll be replaced by a generic levelled draugr boss.

I know that. This instance was the first time I entered the cave. By all accounts, I fought Halldir. What was odd was the fact that his name was not marked as Halldir when I fought him, it simply read Draugr Scourge.