using the Primitive Club on a creeper will cause it to create a stun effect cloud when it explodes

Possibly WIA - Creepers with a potion effect leave a lingering cloud of their potion effect when they explode, and since Stunned is classified as a potion effect (despite the source not being a potion), Stunned Creepers that explode will leave behind a lingering cloud of Stunned. This is technically expected behavior, but a possible oversight of how the Stunned effect is handled.