This is a very rare bug that I couldn't reproduce after I stumbled upon it. Thankfully I was able to record it happening.

In case any of this info helps, I will try to describe how I found it. In a Pillager Outpost, I was making a sweet berry bush farm, then a Pillager and a Skeleton was about to walk into the structure. When the skeleton came in the Pillager pushed the skeleton into the berry bushes while it was trying to walk around them.

Then this happened: https://drive.google.com/file/d/1782zOz1yuFThSOg97fDukjWsemRcugW_/view?usp=sharing

It didn't take damage after moving around like that until I touched it. Very peculiar.

The best way I could recommend to reproduce this is to:
1: Find a Pillager Outpost
2: ALMOST cover it with sweet berry bushes
3: Make sure it is day time
4: Spawn an Un-dead mob
5: Observe as the mob tries to escape the sun into the Pillager Outpost.
6: It should get stuck here as it tries to avoid it while entering the shade:

7: The result should be something like this: