When reporting issues related to item loss, please clarify how you are using the plugin. For example, if you have configured the plugin to keep items in the backpack even when the player dies, then item loss would be surprising, but if you have configured the plugin to drop backpacks on death, then item loss is generally expected as other players could loot it or the container could eventually despawn.
There was once an issue where the player corpses would fall on top of the backpack and occassionally push the backpack under the map, but this was addressed in the plugin by disabling collision between the backpack and its owner's corpse.
There could be other cases where backpacks can be pushed under the map, such as by other objects. You can keep an eye out for this root cause by checking under the map to see if there is a backpack floating in water, and check the server logs to see if an object was destroyed out of bounds near the coordinates that the player dies (e.g., at the same X and Z coordinates of the player, but with a large negative number for the Y axis). If this turns out to be a confirmed issue, there are things I could do to mitigate it, but I'll be waiting for server owner feedback/data to indicate whether this is an actual issue.