Minecraft version: Java 1.16.3, uncertain about earlier versions.

The issue mentioned in the summary above seems very similar to the already reported and currently on status: "resolved" issue, which also is affected by this (no longer resolved!):

MC-91152

The difference to that issue is that fish (cod) are still dropped, but no salmon/clownfish/pufferfish.

For the wiki still sais that guardians would drop the other fish types occasionally I was surprised to find only cod in my guardian farm as well as from killing them with a looting III sword, so far I got 2 doublechests full of them and nothing else.

I have left a note in mentioned issue MC-91152 a few days ago, but for no reply was given I was not certain if the note was seen and the issue would reopen, which is why I created this new bug report, I hope this is ok.