There is an extra key named D with a diamond in the recipe that is not used in the pattern.

This one here needs to be removed
"D": { "tag": "forge:gems/diamond" }

Here is the corrected version and sorry for not makeing a PR out of it:

{ "type": "minecraft:crafting_shaped", "pattern": [ "RRR", "RSR", "RRR" ], "key": { "R": { "tag": "forge:cheese" }, "S": { "item": "minecraft:saddle", "data": 0 } }, "result": { "item": "rats:rat_upgrade_basic_mount" } }

recipe is fixed in the upcoming 1.19 version, you can make a datapack and fix it yourself in 1.16 if you still play that version