Encountered in the Enigmatica 6 Modpack (1.16.4).
You can move the quantum bag into it's inventory if you use the hotbar buttons.
https://i.nuuls.com/x308E.gif

Can duplicate same issue in Direwolf20 1.16. Bag moves into its own inventory, instantly closes and disappears.

same in TNP limitless (QS 5.3.0 Alpha5)

has anyone already found a solution to get the items again?