The bug

Witches that have drank a fire resistant potion will no longer despawn if they are farther than 32m from a player but less than 128m from a player.

Hostile mobs normally despawn over time when farther than 32m but less than 128m from a player. Witches that have not drank a fire resistant potion will follow these rules. But if the witch drinks a fire resistant potions it will no longer follow these rules.

How to reproduce

1. Place a witch inside a glass box with it standing on a magma block or lava. (see picture)
2. Move the player 40m away from the witch.
3. Wait indefinitely for the witch to despawn.

May also work with any potions the witch drinks.
These witches will still despawn if the player goes farther than 128m from them.

Code analysis

Code analysis by gnembon can be found in this comment.