I originally report it here: Deepacat/Crafting-station-improved#1

If a slot of Drum (not tested backpack yet) contains more than 64 items, the slot is possibly invisible or with a wrong count number in the crafting station side inventory.

With my test, this only happens in MP.

Invisible Slots

This is first discovered in GregTech Community Pack, where I added Sophisticated Storage (and its Core) additionally.

GTCPM: 1.14.0
Sophisticated Core: 1.2.80.1073
Sophisticated Storage: 1.3.58.1224

Wrong Number Slots

This is discovered in dev environment of Crafting Station Improved, where in breakpoint, the ItemHandler returned a 64x stone.

Crafting Station Improved: dev
Sophisticated Core: 1.2.80.1073
Sophisticated Storage: 1.3.58.1224

Where BigSlot itself doesn't override the behavior of SlotItemHandler, and it just simply delegate it to the IItemHandler obtained from the capabilities.

So I suppose this bug is caused by Sophisticated Storage (or its core) and report it here also.

Also Affect Backpacks

LMK if you need further details.

Please don't double report like this, wastes time of mod devs, just report where you believe the issue is more likely.
This is clearly happening in the Crafting Station Improved's code and I definitely don't have time debugging and fixing someone else's mod. If they find out that the handler for some reason is returning incorrect info they can provide that info and I can take a look, but that would be surprising given that the same handler is used by pipes / other systems that work with inventories and other mods don't have issue here.

Also the issue they have is most likely syncing of the slot contents to client because by default mc is only able to sync slots with <=128 count

Sorry, I thought it was sophisticated's bug, so I reported it here, thinking you should know this better than me, and actually it is. Thanks for your information about this thresold.

Just to be clear the bug is in Crafting Station mod, they need to adjust the syncing of the slots because it is their gui, I can't change what they do there from my side

Just to be clear the bug is in Crafting Station mod, they need to adjust the syncing of the slots because it is their gui, I can't change what they do there from my side

Thanks anyway. With your helpful hints, I can fix this weird bug. I don't know that the game syncs itemstacks whose count is a byte range, so I can't understand why this happens at the first time. I'm very sorry if I wasted your time.