Steps to reproduce :
- In the nether, build a 5 tall box (3 tall inside). It works best over a very large lava lake, or inside a spawn proofed nether fortress bounding box.
- Inside the box place line of block on the ground and on the ceiling every other block. this should create some sort of 3x1 lanes, with nothing in the middle block.
- Step away from the build and wait for mob to spawn, kill them when they spawn, you eventually will get a large and/or medium magma cube in there.
Only small magma cube and other nether creature should be able to spawn in there.
I've noticed that most of the hitboxes and spawning mechanisms are broken in bedrock, just like spiders spawning in 1x1 area or zombies spawning under trapped doors.
This is really limiting players in terms of smart mob farm contraptions