Placing creatures via Bucket of <name> doesn't properly give them the persistent tag, leading to them quickly despawning. This was tested in 1.20.4 and 1.20.6 and affected both versions.
The fish didn't spawn with the persistent tag regardless of survival or creative, or whether placing the animal into water that was already there. It doesn't matter whether the item was obtained from the creative inventory, or was an empty bucket that later picked up a fish. It affects buckets of cod, salmon, tropical fish, pufferfish, axolotl, and tadpole.
Steps to reproduce:
Place a bucket of <creature>.
Run /data get entity @e[limit=1,sort=nearest,type=!player]
Note that the tag "PersistenceRequired" is equal to `0b` instead of `1b` as it should be. (The tag "FromBucket" is `1b`, as intended.)
I cannot reproduce bucketed fish despawning