Bug description and reproduction steps

Introduce an item(s) then close and open again the backpack (only tested with the larger one).
Mods used:

Log files and Screenshots

No logs because the file doesn't contain any data about this mod and also because is too large due to errors of rendering of Distant Horizons.

Used minecraft version

1.20.x

Used forge or fabric version

Forge - 47.3.0

Used mod version

2.0.1.124

This is also a way to make a "dupe" but i still don't know when it happens, probably because the backpack doesn't save the content data in time.