The bug

Mobs that fall slower than normal, such as chickens, will behave unexpectedly when assigned certain values for gravity.

How to reproduce

1. Summon a chicken
2. Check its gravity attribute - it should be 0.08

   /attribute [UUID] minecraft:generic.gravity base get

3. Set it to -0.08

   /attribute [UUID] minecraft:generic.gravity base set -0.08

Expected results

The chicken would fall upwards at the speed it normally falls downwards at.

Actual results

It falls upwards much, much faster than expected.

Further notes

It may be worth implementing attributes for nonstandard falling speed, such as falling acceleration and terminal velocity, rather than hardcoding them to entities like chickens that results in weird behaviour like this.