Mobs that are standing in water, such as those that would be in a mob farm, do not consistently have the flame effect applied to their drops. At best it's about 50% of the time, at worst it's around 25% of food drops that have the flame effect applied.

For instance see below images. All tests, except the last couple, were done with a Sharpness 5 Fire Aspect 2 Netherite Sword.

This first one I'm not sure how many cows I killed, likely around 30-40.

This second lot is exactly 20 cows worth.

And... 20 sheep

And 20 chickens, just to be thorough.

And... 20 cows with Looting 3 added into the mix

Expected outcome that applied up till around 1.17.30, 100% of mob drops, when one hit killed or just final blow killed with a Fire Aspect sword, would drop their cooked variants even when standing in water or rain.

Did some tests and cannot predict when the drops are going to be cooked or not, all drops are either/or and never mixed so at least it's specifically dependent on whether the mob is "officially" on fire at time of death or not. Regardless of whether they sound or appear on fire btw.

Just spawned a bunch of cows between a water logged lower slab and a solid block (1.5 blocks of standing space) such that they weren't able to bob any significant distance and... I got the below results. Clearly it doesn't matter a whole lot how much water the mob is standing in. : already retested with a full block of water below and a upper half slab above head... 100% raw drops, same outcome with rain, the rain test was accidental...

Issue is replicable both on a world upgraded from 1.15 to 1.16 to 1.17 as well as a 1.17 currently .32 server and a fresh 1.17.34 creative world.