I got the same issue. When creating a world, you have to go to the multiplayer tab on the left and disable "multiplayer game". That should fix the problem
No resources Packs. It also happens when stand on a raft instead of a slab
It does. But this issue happend the 3rd or 4th time
I tested the -395/ -24/ 1229 on mobile and no mobs spawned.
The trial chambers where the Mops spawn, have been looted before with the intended mob spawning behaviour. The issue only appeared after the latest Update.
Seed -1674244446
-395/ -25/ 1220
-398/ -18/ 1206
-282/ -14/ 1973
-345/ -28/ 1946 (marked and enclosed the slime chunk, see pic above)
...
I checked more trial chambers and - as far as I noticed - I think only trial chambers generated before 1.21.21 spawn mobs. The one trial chamber i checked in newly generated chunks (I checked it twice) didn't have any ordinarily spawned mobs.