what is the cause of the backburner's inconsistency with crits from behind?

what exactly is the reason for why backburner crits are sometimes inconsistent? people always regurgitate this like it completely ruins the weapon (it doesn't) but I do sometimes notice not getting crits when I should have or getting crits when i shouldn't have, but I've never had a clear answer on this besides "source spaghetti, flame particles jank" or something vague like that.