Equipped backpacks from "Sophisticated Backapcks" not appearing in grave, but rather drops next to grave

[MathiasFurenes]

Version:
Gravestones version: v.1.15
Sophisticated backpacks: 1.20.1-1.2.7.15.166
Minecraft version: 1.21
Fabric version: 0.18.2

Describe the problem
When you die, backpacks equipped (specifically from "Sophisticated Backpacks" by "P3pp3rF1y, Ridanisaurus and Salandora" in my case) drops next to the grave instead of going inside it. That means you have exactly 5 minutes to find it or it will despawn, which negates the whole point of the gravestones mod. Backpacks in your inventory do go in the grave.

Steps to deproduce

1. Equip a backpack
2. /kill
3. Observe how the backpack drops as an item