Players and mobs don't take vertical knockback when already in the air

The bug

When you hit something, it will fly backwards and jump up into the air slightly. Normally, if you hit it again when it's already been flung into the air, it gains more backward and upward velocity. In the current version, the second hit will only cause horizontal velocity, and hit the ground at the same time as if it didn't get hit.

The bug doesn't happen every time, so don't close this after testing it once without it happening. I'm not sure exactly what causes it to work or not, but everyone I've talked to has had it happen to them. It seems like it happens more often with players than mobs.

How to reproduce

Video demonstrating the issue: https://youtu.be/TRe-OVh04yg