Something about frozen mobs is related to this bug.
Also this is on 1.16.101 Hotfix on the Nintendo Switch.
Adding on to this the entities are molded at the wrong position.
The module stops moving while the hit box does.
/tp @e ~~~ works on the entity but not the module.
It happened to me in a flat world while I was testing the MCPE-60895 bug.
I believe it may have happened because the entity cliped throw a block.
I found the mottle inside a block and the entity outside of a block.
Also the mottle disappears when the entity is killed.
Also the mottle still does the animations of a chicken falling.
It starts by moving, sometime later it stopped.
This works with almost all full blocks and many partial blocks.
This continues to happened until you let go of the block placing button.
It seems to have a affected range relative to the players y position.