Poison, Wither, Fire and Cacti damage deal vertical knockback to players

The bug

Taking damage from status effects (poison/wither), fire or cacti will deal vertical knockback to the player, even though no knockback should be expected.

How to reproduce

• Go into Survival mode and apply Poison II to yourself
• Attempt to jump up a fence, a block with a bottom half-slab on top or an equivalent height

The poison damage will allow you to jump up the 1.5 block height, even though this jump cannot be performed normally. Also note that sometimes taking damage will momentarily "pin" you to the ground and prevent you from jumping at all.