This bug is easiest to reproduce with the Sophisticated Backpacks mod, and when I contacted the developer of that mod he had said that this bug was on Better Combat's end

Minecraft version - 1.19.2
Mod loader - CursedForge (?)
Mod version - 1.7.1 + 1.19
I am using the latest version available - No

Describe the bug
Items funneled into the offhand slot when holding a two handed weapon are deleted

To Reproduce
Note: Uses Sophisticated Backpacks mod
Steps to reproduce the behavior:

1. Obtain advanced refill upgrade.
2. Place it inside of the backpack's upgrade slots (the bar on the side)
3. Fill the bag with any item
4. Click on the tab on the right that says "Advanced Refill"
5. Select the item and click on an empty slot in the tab.
6. Set it to refill to the offhand
7. Place the bag in your inventory
8. Select two handed weapon
9. Quickly open the backpack (right click it if it is in your hotbar or press B)
10. See the items get funneled into the void

Expected behavior
The offhand slot is not considered an empty spot for items to be funneled into

Screenshots or video recordings
(https://drive.google.com/file/d/1jk4bQ2OZng9D0xaeume5GWM3DGMQYatd/view?usp=sharing)

Additional context
More of a concern but I don't know if this bug might also cause a memory leak since the items may not be properly deleted

This is caused by Forge mod loader introducing breaking changes retrospectively.
I recommend stepping down the version of FML.

1.19 is a very old game version, there is no development capacity to run after every malevolent change Forge creates.
The primarily recommended loader is Fabric.