I tried searching for a similar problem, but could only find old ones, like: MC-184740

This is basically my exact issue, I wanted to test rates for 1.17 vs 1.18 because i heard 1.18 spawn rates are bad. (Formula for sub-0 Y-level spawn rates should make < 0 more common, leaving existing spawn rates unaffected)

Anyway so I loaded my world in 21w41a, and my witch farm was full of spiders and no witches.

Expected behavior: Only witches should spawn within the (now demolished) structure.

Observed behavior: 'all' mobs spawn in the chamber, virtually no witches.

World created in: 1.17.1, chunk generated in 1.17.1

Chunk details: see screenshot