Okay, this time I did get XP from an enemy being killed by burning.
Either this is inconsistent, or it got patched.
In later case, either I just didn't notice before, or something else is going on.
That being said, the game does act a bit weird if I load a slightly earlier file by sometimes removing Items from my inventory or not loading context-sensitive dialogue (such as when the characters get '!' above their heads). I wonder if this has to do with me installing it on an external disk?