The devs decided to change the knockback of uppercut and downslam in Project Smash Roblox. This video showcases it, what it means for you, and how to adapt/work around this mechanic.
If the person is directly below you and its at the void then you can still knock them (also get people close to the edge and downslam because it knocks them back to the void) hope it helps. lov ur content :)