I’ve killed this thing six times in a row. Each time, it breaks up and despawns on death, but I get no credit.
I did manage to kill it yesterday and there were body parts then after the kill. The animation has since changed and the creature just despawns on death.
I believe due to how it’s coded you need to kill him first, after which his model disappears and then you can hit the “invisible” corpse three more times for credit. At least it is how it worked for me a few days ago.
There’s also an achiev which can trigger by hitting his corpse only once for 2500 score (legendary catch), after which you have to instantly cancel the dive.
Other players complicate things but killing him alone shouldn’t be hard over 2-4 dives, depending on how consistently the harpoon will hit.