Kog Maw R not applying Black Cleaver stacks

[Gameplay] Kog Maw R not applying Black Cleaver stacks

I was trying out some builds on Kog Maw and noticed his R does not apply a stack of the Black Cleaver even though it does deal physical dmg. I think this may have been something overlooked when they added the physical dmg to his R awhile back. I tried this in custom games in regular and PBE.