Sorry, i missed your answer.
I reloaded my save, and could not reproduce the bug. :/ (i got xp every time, killing him with different means)
I think the problem with gas grenade and molotov lingering flames is more important and reproductible. It's like the game doesn't know this gas "belong" to you, so out of aggro range, enemies just die standing in the gas, and you get no xp for it. not link to my level.
PS: Can't wait for the DLC, i'm playing other solo RPG in the meantime and it makes me realize how good the balancing of your game is compare to many other, so many viable build even on DOMINATING, but still challenging experience, really great job !