Describe the Bug

I think a video illustrates this best (below), but in short, if I place a hopper on top of a toolbox and then very quickly throw many different items into the hopper, some items appear to be getting transformed into other items that were in the hopper. My guess would be that this is related to the custom storage slots that can go up to 256, and to zero without loosing the item assigned.
Do note that whenever an item converts, the type of item that got lost in the conversion to another (e.g. when dirt turned into nether star, the dirt) did not get a permanent slot assigned in the toolbox. When I shift click out all the items, all others keep a zero stack, but that particular one does not. This may be related to the cause of the issue here.

Testing in a modpack:

Testing in a minimal modset, only create fabric and mc

And here's a log from the minimal modset:
7iexZoi.txt

Reproduction Steps

see videos

Expected Result

no items should be converted

Screenshots and Videos

see above

Crash Report or Log

7iexZoi.txt

Operating System

Win10

Mod Version

0.5.1f

Minecraft Version

1.20.1

Other Mods

No response

Additional Context

No response

1.20.1 only ~ doesn't affect forge and doesn't happen on any other version

Also, does not only work with hoppers, but any other form of item input. Tested with chutes, and all items were converted to 1 type.