[Edit: I'm struggling to get onedrive and google drive to work to upload the world file for the world where this works.

This also only happens in the lukewarm ocean around my base, not in the regular ocean a couple hundred blocks away.

Edit 2: It only works in my survival worlds seed.
The seed is -8365520427056912195, bedrock edition]

If you go to a one block deep any water source and right click on a fish with an empty bucket (This may involve putting your head inside the fish, the living entity will cease to exist. No bucket of fish will enter your inventory.
I was only able to see this happen in my survival world. I haven't been able to replicate it yet. Even with the exact same inventory, it only happened in the survival world.

I only tested it with cod, salmon, tropical fish, and axolotl, and all of them just disappeared