After making a witch farm on a server I play on, I discovered no witches were being spawned.
After investigation, it appears that the witches were attempting to spawn on the corners of blocks and not the center, and thus colliding with components of the farm, preventing them from spawning
you can easily reproduce this using a similar setup as shown in the attached video, the world seed used is -1670315946175533161
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I haven't had a chance to test this in 1.16 yet but I will do so later today. Additionally, I have also discovered since reporting this that it is not limited to witches. It occurs for all hostile mobs in the effected area, and I haven't had a chance to observe other mob types thus far. My guess is that it's something to do with how far away I am from spawn, but I've got no idea what the lower bound is for this occurring.
I don't know if this occurs in previous versions, but the sheer number of previous versions to test it in makes me reluctant to try, although I might give 1.12, 1.13, and 1.14 a shot on their most developed releases.
Finally, some interesting observations:
Mobs spawn specifically in the north west corner of the selected block for spawning.
Even if the selected block for spawning is an air block, the mob may still spawn given that one of the three other blocks that shares a corner with the north west corner of the spawning block is also a valid spawning block. This particular observation isn't definite, but is at least a representation of what happens visually
Also not a definite observation, but mobs only appear to be selecting every second block for spawning. That is, only if both the X and Z co-ordinates of the block are divisible by two. If this is consistent, it implies that only 25% of all of the blocks are spawning mobs.
Again, these are not concrete and proved, but are at least consistent with what happens visually. I will try to record some examples of these observations.
Has this occurred in previous versions of the game? Could you also please test it in 1.16?