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.
