I tried this Plagg, Nik-Nuk, and Crub fight some more times and figured out the trigger:
If Wyll has joined you but isn't in your party when you kill PNC, the game breaks your oath -- regardless if you've ever talked to Wyll at camp or adventured with him. I reloaded, added Wyll to my party, talked to him at camp, and had him with me to kill PNC. Oath remained intact.
That's a weird way to break your oath or not. Looks like some oath breaking trigger is mixed with companion's quest flags maybe.
That does sound odd. I happened to have a save just before this fight so gave it a try. I do not break my oath either when killing PNC with Wyll in my party, or if I dismiss him back to camp and then kill them without him. So perhaps there’s a weird difference between the oaths (I’m playing devotion) or there’s some further subtlety behind this bug beyond simply whether Wyll is in your party.
In case any of this is relevant, I have had Wyll in my party for most of the game, and have spoken to him multiple times in camp up to him telling me all about Mizora. He has a High opinion of me. My save is also post-party, as I’d actually forgotten to mop up PNC in my original clear out of the sanctum.
