In 1.20.70.24 Preview,You Can't Find Any Monster Other Than Bogged And Slime In Swamp Biome Cave.
I Also Test In Savanna, It Spawn Monster As Usual.
I Has Set Difficulty To Hard,I Also has Disabled My All Pack.
I Also Test My Android Device, It Also Affect Too.

How To Reproduce?
1.Create A World And Make Sure The Difficulty Was Set Hard, Then Go To Mangrove Swamp.
2.Try To Leave Other Biome Other Than Mangrove Swamp.
3.Use /tp to Teleport Some Monster Like Zombie And Creeper To You, You Will Find No Monster Was Teleported.(The Chat Will Show Red Text) And Then Use /tp to Teleport Bogged To You, You Maybe Can See Bogged Was Teleported.