Oil slicks in BG3 are very touchy and make absolutely 0 sense even in terms of patented Larian surface mcguffins.
I don't doubt there are countless additional ways besides torches and flaming swords to inadvertantly trigger an ignition.
Heck, I wouldn't be surprised if merely the act of thinking about fire could set it off.
So, I'd double check every possibility, regardless of how much sense it makes, to try and figure out the cause of your ignition issue.
