Describe the bug
I have been hosting a multiplayer server that runs my modpack. For some reason, I am having issues where upgrading to netherite backpack from diamond one, I lose all my items in the bag. It's not every time, seemingly happens at random but it's broken currently. Multiple people have reported this bug on my modpack. No logs are provided that show any error anywhere.

To Reproduce
Steps to reproduce the behavior:

1. Obtain Diamond Backpack, fill it with contents
2. 2. Upgrade the diamond backpack to netherite
3. Lose backback contents at random chance

Expected behavior
Upgrade succeeds without fail.

Versions
SophisticatedBackpacks 1.21.1 3.21.1.1.32
Fabric 1.21.1 (0.16.10 + 0.16.11)

for reference, here is my modpack: https://www.curseforge.com/minecraft/modpacks/spawnpoint-client-fabric/files/6135423

Checked it against the version mentioned and with many backpacks and I can't reproduce it.

Are you using anything to change the upgrade recipe?

Upon further investigation it appears Easy Anvils (or its dependency, puzzleslib) was causing me to crash whilst using the anvil upgrade. (Unrelated to OP, but worth mentioning since it was on my to-fix list.)

As for the losing backpack data, I have also not been able to reproduce it with the aforementioned uninstalled. It may very well just be an incompatibility that I completely missed in hindsight, but might be worth looking into.

I have used AnvilNeverTooExpensive as an alternative, with no crashes using the upgrade, or losing backpack data since.

Also, no recipes for upgrades or anything regarding recipes were altered.