Grindstone repairing does not work

[sciwhiz12]

Minecraft Version: 1.16.1

Forge Version: 32.0.57

Logs: not applicable, in-game issue

Steps to Reproduce:

1. Obtain two identically damaged diamond pickaxes (/give @s minecraft:diamond_pickaxe{Damage:768s} 2)
2. Put the two items into the grindstone's interface
3. Observe that there is an invalid result (non-stackable item is stacked and not repaired)
4. Break a block with one of the diamond pickaxes (so they are not identically damaged)
5. Put the two items into the grindstone's interface
6. Observe that there is no result

Description of issue:
When using the grindstone to repair two tools of the same item, the result is bugged and not correct.

The correct result with the given steps for both 1-3 and 4-6 is a fully repaired diamond pickaxe.
The actual result from following the given steps on the given Forge version differs:

• Steps 1-3 result in a stack of the given pickaxes, with the count of 2. (also happens with enchanted pickaxes)
• Steps 4-6 result in nothing.

Original bug report on forums (this is one part of the report)

Screenshot of result of steps 1-3 on given Forge version

Screenshot of result of steps 1-3 on given Forge version, using enchanted pickaxes

Screenshot of result of steps 4-6 on given Forge version (damage values are exaggerated to visually differentiate the items)