Fish that die on magma drop cooked fish. Other mobs don't. I believe that this is a bug because magma shouldn't do this and it is annoying (at least for me).

How to re-create:

1) Make ray's fish farm (https://www.youtube.com/watch?v=FQvRS1_64QA).

2) Either AFK at the farm or spawn in fish using spawn eggs/buckets of fish.

3) Go to the collection and see that the fish are all cooked.