Why is this whiffing?

I always get hit/hurtbox messed up...

Hurtbox based, this happens in every fighting game. You should've tested in Dojo to have a perfectly flat surface too for better tests btw.

his P+K got weird in several regards. for instance if you try to do P+K, roll P it has 50% chance to completely whiff depending on if its closed/open stance. not to mention all the force tech stuff, sometimes it works sometimes it doesnt.

This happened in the change from vanilla to Ultimate (this is not a new thing). It's not weight class based but more hitbox size dependent. P+K used to be his best force tech into mixups and somewhere in the game change it was lost. 8P is the go to now unless you are going uphill.

Bass ain't fat, he's seasonally plump.