[Kameron8]

The Deadly Smile weapon potential, Jolly Slaughter, is not currently removing Knockdown Immunity from mobs.  To replicate:

1.  Equip a Deadly Smile with the potential unlocked.  Enchant with Haunted Soul for ease of testing.
2.  Fight a Prinny.  Hit it with the Deadly Smile to knock down the Prinny.  End turn.
3.  Once the Prinny stands up and gains knockdown immunity, use Jolly Slaughter to attack again.
4.  Note the Prinny is not knocked down, and still has knockdown immunity.

I tested this against Snakemen as well in the off chance this was a Prinny specific issue, to identical effect.