The bug

For shifting floor design farms it's possible for mobs to get stuck inside the soulsand block if they are positioned between two slices when the floor shifts. The mob can get into this position if mobs from above floors fall on them and nudge them over. This wasn't an issue in 1.13.

To demonstrate, I've made a short video showing off this behaviour: https://youtu.be/45KmI_1q2xk. I've also attached the world shown in the video to this bug report.

The reason (from MC-203361)

In addition to the top collision box at 14px from the bottom, soul sand has additional collision boxes at 12px, 10px, 8px, 6px, 4px, and 2px from the bottom.

8, 6, and 4 snow layers have the same extra collision boxes (when applicable) while 7 snow layers has them every 4 pixels.

How to reproduce (from MC-203361)

1. Place soul sand or snow layers
2. If using soul sand, surround it 1 block above with another block (the soul sand will push the player out otherwise).
3. Stand on the soul sand.
4. TP the player down through the top collision box of the soul sand with the command

   /tp ~ ~-0.01 ~

5. Repeat this command for each additional collision box.

Observed Results:
The player stops falling and stands inside the soul sand at heights of 0.75, 0.625, 0.5, 0.375, 0.25, 0.125 for each use of the command before eventually falling fully through the block. For 7 snow layers, they stop at 0.5, and 0.25. For 6 snow layers, they stop at 0.5, 0.375, 0.25, and 0.125. For 4 snow layers, they stop at 0.25, and 0.125.

Expected Results:
The player would fall completely through the block to match the behavior of other blocks, including other non-full blocks like honey blocks.