Describe the bug
Shift-clicking an item from the crafting grid to a partially filled slot in your inventory with a stack count that exceeds the target stack drops the difference in stacked items while keeping it in the Crafting Grid slot.

To Reproduce
Steps to reproduce the behavior:

1. Fill backpack and inventory with any items
2. Place 32 of the items you want to duplicate in a slot in your inventory
3. Place 64 of the items you want to duplicate in any of the backpack's Crafting Grid slots
4. Shift-click the stack of items from the Crafting Grid slot
5. 32 of the items are transferred to the inventory, 32 items remain in the Crafting Grid Slot, and 32 items are dropped from the player.

Expected behavior
Only up to the stack count of items are placed into the inventory, the rest remain in the grid.

Screenshots
Screenshot Album

Versions
Minecraft 1.16.5
Forge 36.2.35
Sophisticated Backpacks 1.16.5-3.15.15.607
No other mods installed.

Is sophisticated backpacks the only mod in your modpack? If not have you got fastworkbench without replacement in the pack? That is cause of your issue in that case

As I mentioned I have no other mods installed. I noticed the issue in a larger pack, so I created a new setup where that was the only mod and got the most recent version to confirm it wasn't some conflict.

This is fixed in the latest release on curseforge