When you right-click on a waterlogged block with a fish bucket, axolotl bucket or tadpole bucket, the mob will appear above the block. If there is another block above the waterlogging block, the mob will get stuck in it.

How to reproduce:

1. Give yourself a fish/axolotl/tadpole bucket, a leaf block, and another block.
2. Place the leaf block and the other block on top of it.
   
3. Use the bucket on the leaf. You will see the mob stuck in the block and suffocate to death.