It seems that if you have another item in the back slot and you right click on the backpack shelf, the item in your back slot will be deleted.
This happened to me using a satchel from the scout mod and right clicking on the backpack shelf.

edit: everything inside the backslot backpack or satchel will be also deleted
edit2: this was on fabric version 1.20.1-3.0.0-beta.2

oh god, I just discovered this is the case with the quark oddities backpack, too.
1.20.1