1.21.1 Mob spawns are insanely high

[FlyingVillager]

Minecraft version

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Describe the issue

I help run a multiplayer server for me and a handful of friends and everything seemed to work fine except for the game breaking amount of passive mobs spawning. We noticed issues earlier on but though it was just because we were in one spot for a while but after less than a day of playing there would be 2000-3000 passive entities and it would cause the server to fall hundreds of tics behind even if there was only a single person online. We would cull all passive mobs and less than an hour later it would be back in the thousands of mobs. The only fix was to kill all the mobs and remove the mod and haven't had any issues with that since.

We did not have any portals running at that time either so we're at a complete loss as to why it had such an impact.
Did it do this before the recent updates?