while mining into a nether cave, i discovered a few full sized magma cubes stuck 1 layer under the surface leading to the mob not efficiently be able to agro to the player. 

discovered in my realm. seems to only happen in 2x2 caves or mined areas.

Reproduced on realm - 10/10

Reproduced on backup copy of realm - 10/10

Steps to reproduce

1. Boot Minecraft bedrock
2. load into world of choice
3. enter nether
4. mine through the netherack making it a 2x2 space from floor to ceiling. 
5. run out of the visible location and return after reaching spawnable distance.
6. locate 2x2 magma cube.
7. observe full sized magma cube stuck in the floor of netherack.