Minecraft v1.18.1
Forge v36.1.2
Quark r3.1-340
Sophisticated Backpacks v3.15.15.550

After adding net.p3pp3rf1y.sophisticatedbackpacks.client.gui.BackpackScreen to the allowed screens option for inventory sorting, I discovered that sorting any sophisticated iron, gold, diamond, or netherite backpack with a full or partial full inventory will delete many items.

I had enabled this so that items could be easily put into the backpack using the insert button.