As of Identity 1.10.0 for Minecraft 1.16.3, if you are disguised as an undead creature (including the Wither), and you aggro a Witch, the witch will throw splash potions of Harming on you even though they should heal you. Witches should throw splash potions of Health on players disguised as undead (especially if #66 is fixed), as that will fix a healing exploit involving disguising yourself as an undead and letting a Witch attack you.
To reproduce:

1. Get your health below full but above 8 HP (e.g. half vanilla HP).
2. Apply the zombie identity to yourself (or any other undead)
3. Aggro any Witch.
4. The witch will throw splash Potions of Harming on you.
5. You will be healed from the Instant Damage effect they cause.