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
/summon chicken ~ ~ ~ {Passengers:[{id:husk,IsBaby:1}],IsChickenJockey:1b}
Look at the chicken
Press F3 + I
Open a command block
Paste the command inside and activate it
→ ❌ Keeping entity minecraft:husk that already exists with UUID <...>
Credit
Found by @unknown in this video: https://youtu.be/uJVkdTpdI2o?t=429
Confirmed in 1.16-pre2.