On my realm I live on a mushroom island. I have noticed on occasion close to the ocean, mobs will spawn within the mushroom biome itself. This isn't the same as MC-93271, as I had already removed all of the land from the ocean biome itself (see screenshots where I screenshotted the mob right after it spawned and then the biome/coordinate information at that location).

Seed: 494 (World first generated in 1.17 singleplayer)

To recreate, you can either wait for the mobs to spawn or fly up and down to speed up the process like I did.  Note: I was unable to recreate this in singleplayer, thought I don't think this is exclusively a Realms issue, but rather a multiplayer issue.

I'm not sure if this has anything to do with pack spawning or not.