Oil Grenade + Detonator Skills + Whirlwind didn't work!

Hello. I don't know why it is not working.

Whirlwind in skill Whirling Slash said can overlap elemental surface.

However, Oil Grenade create oil ground and can create ignite when Detonator Skills use on it e.g. "Explosive Grenade", "Explosive Shot", "Explosive Spear"

Oil Grenade create ignite ground correct, but Whirlwind don't overlap ignite on Oil Grenade + Detonator Skills.

Is it a bug?