Witch huts with normal mobs.

I was playing on an smp/anarchy-ish server, (mc.andrewsdatacenter.com). And I made a witch farm. Except it spawns no witches! Only things like skellys zombies creepers spiders etc... and yes I built the farm in the structures bounding box and the server has only been running since 20w06a and I also tested the seed in a single player world in that version and the witch hut WAS there! So I am confused as to why my witch farm that should be working, just spawns normal mobs. Also if you read this thank you you are awesome and have a great day you guys at Mojang do such a good job I love your game