PaperSpigot 1.13.

When a player has the permission backpack.keepondeath it is keeping their regular inventory as well as the backpack. I believe this is only supposed to keep the backpack inventory.

From SpigotMC page with permissions descriptions.

Please try if this build resolves the issue for you: https://ci.pcgamingfreaks.at/job/Minepacks/68/artifact/target/MinePacks-1.17.13-SNAPSHOT.jar
If not please provide me with a list, of the plugins that you have installed.