Can confirm experienced on unmodified multiplayer server with unmodified clients on snapshot 21w10a with two players and low latency.
Reproduction steps:
1. Create new world
2. Gather materials for a stone pickaxe in survival (Two sticks and three cobblestone)
3. Craft a stone pickaxe, ensuring that it is shift clicked to send it to inventory
4. Use the pickaxe, either by moving it around in the inventory or mining a block with it. The advancement will only trigger then.
Additional: it has come to my attention that the behaviour of rabbits matches those of chickens in dropping one of itself unless looting is applied. This means that there is an in game trend of animals dropping one item has an exception allowing the drop of more than one only with looting. This further proves fish mobs have inconsistency and therefore bug like behaviour
Weird. The Jira migration deleted all my comments I guess.
tl;dr behaviour is inconsistent and the ‘logically they’re the whole one animal' rebuttal doesn’t make sense - see chicken, rabbit. Both are whole carcasses but both mob drops are affected by looting.