The bug

When guardians try to swim in one block filled with water, it leads to strange behaviors such as not moving after removing the water. This is because they are swimming in one block of water, (Which never happens in other versions like 1.15.2.) causing their Motion tag to get a NaN value.

This was never possible in 1.15.2 as guardians never try to swim away from a player when stuck in one block of water.

How to reproduce

1. Dig a one block hole and put water
2. Spawn a guardian
3. Go to survival
   → The guardian starts swimming

4.  /data get entity @e[type=guardian,limit=1] Motion 

   → Chat will display "Guardian has the following entity data " which shows the NaN value.