What I expected:
A pig killed by fire/lava damage (for standing in a fire/lava block) no matter the weather should drop cooked porkchop.
What happened:
While it was raining a pig was running through a wall of burning netherrack and died just as it was about to leave the fire zone. It dropped raw porkchops.

It seems the actual trigger for cooked meat is the last HP being drained by the fire DoT effect which is put out during rain before dealing damage. Adding damage by lava/fire block as triggers would solve this.