This issue is a partial clone of MCPE-9999. The 1.13.0.9 beta changelog reported that both mob hitboxes and mob collision boxes would be removed during the death animation. However, only mob hitboxes were actually removed. Collision boxes have continued to function after death.

Steps to reproduce

1. Set up a 1-block wide water stream from a single source block, with 2-block high sidewalls.
2. Spawn a few mobs in succession on the water source block. They should be pushed in a line by the stream.
3. Kill the mob at the end of the stream.

Expected result

As soon as you kill one mob, the mob behind it gets pushed into its place by the water.

Observed result

After you kill one mob, the mob behind it does not take its place until after the first mob turns to smoke at the end of the death animation.