Tested on Server and in Singleplayer, Version 1.3.0 (1.3.1 only fixed bag in bag right?)

Step 1 : Get 3 backpacks
Step 2: Open backpack 2
Step 3: Change the place of backpack 1 to the 3

also a suggestion to prevent players from just hording backpacks -> 1 backpack per player inventory config

1.3.1 doesn't necessarily fix bag-in-bag... it just prevents you from interacting with slots that have bags in them. It should prevent this issue as well.

This was how it worked in 1.15 (and I don't recall getting reports for duping or deleted backpacks), but the slot locking was broken in the 1.16 GUI changes, so I think everything should be good now.

Thank you I will try it and report back in 1h

1.3.1 - If you have to bags of the same type they can get identical inventories, and you can take everything out of bag 1 and then the same items out of bag 2.

Okay so I could not reproduce the dupe, but apperently you can still move between backpacks via hotkeys, which deleted a full endless backpack for one player - happens - just wanted to tell you.