The bug

When copying the NBT of a mob using F3+I, the UUID of that mob is not included in the resulting command (see MC-175830), however the UUID(s) of its passengers is/are still copied. When executing the command, the mob itself is summoned in successfully, however its passengers are not summoned in and a warning is logged in the console.

To reproduce

1. /summon chicken ~ ~ ~ {Passengers:[{id:husk,IsBaby:1}],IsChickenJockey:1b}

2. Look at the chicken
3. Press F3 + I
4. Open a command block
5. Paste the command inside and activate it
   → Keeping entity minecraft:husk that already exists with UUID <...>

Credit

Found by Rays in this video: https://youtu.be/uJVkdTpdI2o?t=429