Use CreateTweaker (just how I added the recipe, should be able to reproduce with a datapack just fine) to add the following recipe:

<recipetype:create:pressing>.addRecipe("rottenflesh_to_leather", [<item:minecraft:leather> % 45], <item:minecraft:rotten_flesh>);

When pressing it on a block in the world, you get the expected result, getting leather at around 45% of the time.
When pressing it on a belt, you always get leather, 100% of the time.

JEI shows the recipe correctly.

Video of the bug below.
https://i.blamejared.com/12fgh.webm

MC: 1.16.5
Create: mc1.16.3_v0.3e