This is similar to an issue from 4 years ago DarkPacks/Crackpack-3#25

When you as a player die in the following ways, the backpack loses its contents:

• Dying with the backpack in your inventory and recovering it from your grave
• Dying in lava
• The other ways mentioned in the aforementioned issue.

I have encountered this, too. I died by drowning with the backpack in my inventory.