Cows, Chickens Pigs and others drop cooked meat upon death by fire. Guardians however can die in lava or fire, but don't produce cooked fish. Their dropped fish remains uncooked.

Steps to Reproduce:
1. Create a layer of hoppers with a block of air above it, above the air signs and above the signs lava
2. Let a guardian fall through it
3. See the uncooked fish appear in the hoppers