So was playing normally with my wife on bedrock (Xbox series x, split screen) she had corralled some farm animals into a large barn, separated them out, and had bred lots and lots and lots of chickens, sheep, cows and pigs, with the possibility of slaying or shearing them, some time later we realised that it was acting as a mob switch any time we were near the barn, nothing will spawn, no matter what dimension the other person is in, this was confirmed when we decided to kill the chickens and pigs, things started spawning and it was a lightbulb moment.

Tldr: too many farm animals creates mob switch