Forge: 40.1.31
Voidscape: 1.18.2-0.2.67

When an effect is given to the player in the void (namely wither), when the player respawns back in the overworld the effect persists. In the case of wither that then kills the player in the overworld (wasting any TF charms the player has lol)

Found on the TF official testing server

It's because you dont actually die.

These effects arent obtainable in the void unless you drink the potion for whatever reason.

Wither skeletons spawn in the nether parts of the void, which give wither when killing the player there

Ah so they do, I don't remember adding them but maybe i'll remove them.