How often do Flame Cultists leave flame behind?

[drspendlove]

Sometimes, when you destroy a flame cultist, it will leave a tile of flame behind. Do we know how often this happens and if it is affected by difficulty? I am excluding the case where the cultist dies due to a flame attack of your own.

[fallen]

drspendlove - this should generally happen. There is a small random chance it won't. Not effected by difficulty.