Witches unable to kill undead mobs when attacked as they only use slowness and poison potions to attack them and poison potions aren't able to kill mobs nor even damage undead mobs. All of that results in witches only damaging themselves while not even damaging the mob, just slowing it.
Thank you for your report!
We're actually already tracking this issue in MC-94281, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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