Items disappear after opening the backpack

[peigenr]

Hello, I have problems, I am playing my Minecraft server (Mohist) Under normal circumstances, when there are items in the backpack in survival mode, you cannot use mouse drag to discard it, but you can press the q key to discard, and then no items are dropped, and after taking a new backpack to open it, the items all disappear, and there is also a problem that sometimes inexplicable items disappear completely, which is very strange, such as last night I put many items in the backpack, All disappeared when I played today, and my friend put the loot (equipment and the like) got after fighting dragons (Ice and Fire mod) into backpack and reopened it all disappeared.

[peigenr]

The game version is 1.16.5