Witch hut is not spawning witches. I build a witch farm (3 spawning pads (2 blocks high space with stone and soul sand alternated as the floor) with light level zero under a big half slab roof) in the bounding box of a witch hut. The afk spot is in the center and 120 blocks above the pads and everything except the spawn pads is spawn prooved (aka blockes removed).

If I load up the same world files in SinglePlayer or with Mojangs Vanilla Server, witches spawn without problems, however with spigot, no spawns occur.
So far I tried:

• afking closer to the farm (tried e.g. 58 blocks above the farm)
• setting mob-spawn-range to 8 or 16
• setting entity activation and tracking ranges to 128

Further remarks:

• Other mobs (if I move to spawnable blocks) seem to be able to spawn just fine
• When I join the server, sometimes there is a (set of) witch(es) in the farm, but no new ones spawn
• Spawn-limits for monsters is at 70
• no other hostile mobs are loaded -> mob cap is not a problem

Bootup log of the Spigot Server:
https://pastebin.com/vJ1Vjf59
spigot.yml
https://pastebin.com/kSEaybwj
bukkit.yml
https://pastebin.com/UWq5eCCm
server.properties
https://pastebin.com/FxxCcg0E

World Download
https://gigamove.rz.rwth-aachen.de/d/id/9LK344NJaLXjEr

With the spigot and vanilla jars included for your convenience
The farm is at (  -3355  |   68   |    5379  )