Context Environment

Minecraft version: 1.12.2

Ancient Warfare 2 version: 2.7.0.1032

Minecraft Forge version: forge-14.23.5.2854

If you are using a modpack, link to the pack: Super Hostile Modpack

Expected Behavior

I'm fairly certain that a pack, or rather its inventory, is supposed to work exactly like a vanilla chest. In this case, there shouldn't be any problems in putting an item (or block) in the inventory slot initially occupied by another item/block (i.e. swapping the two items).
Note: by "initially occupied" I mean that it was occupied even before opening the inventory.

Current Behavior

It doesn't let you do as described above. It's as if every time the inventory is opened, the slots containing items/block become temporarily reserved for those exact items only (e.g. you can't put an iron block in the slot which is/was occupied by another block). Even while shift-clicking items into the pack, it all behaves as if those slots were somehow still occupied.
Closing and reopening the inventory resets this behaviour.

If it helps, here's a video I recorded to show this. Note that when my cursor is not moving it's because I'm left-clicking.

Steps to Reproduce

1. Take newly created pack (the size/tier doesn't matter if I'm not mistaken);
2. Open pack;
3. Insert [item1] in [slot1], where [item1] is a certain item/block and [slot1] is a certain inventory slot (e.g. top-left slot);
4. Close pack;
5. Open pack again;
6. Try putting [item2] in [slot1], where [item2] is different from [item1], or put [item1] in a different slot and then try putting [item2] in the now "empty" [slot1] : you cannot.