It works exacly same without resource pack and optifine... tested it and it is same issue... So another post is there.
Is there any way to get this cat? to get advancement? In villages -> obviously don't spawn. In witch hut -> just black cat.. Any other way to get it? In survival without modifing game files or using cheats, commands or mods.
I just finally figured out why they spawned multiple times in my mushroom biome, and it is this exact bug. it can happend in survival mode just too easily, even if player is just AFK and did not built anything before. It can happend with stuff like naturally spawned axolotl killing naturally spawned drowned, spawning zombies on mushroom biome where it shouldnt, even on things like redstone where they shouldn't. In player-build structures like mob trap, it can spawn outside... I just feel like spawn mechanics of reinforcements are just bad as a whole, not just biome itself. it is cool feature, but this spawning not properly implemented. So even biome that shoud be completly monster free is not safe from monsters, both from phantoms and this bug.