It appears that you can (is this intentional?) nest backpacks into other backpacks, which makes for unlimited storage.
If this is intended, there should be an option to disable it.
Otherwise, the backpack recognition seems to be broken, as you can't put backpacks into the combined backpack but any other backpacks.
Infact, you cannot pick up any items when inside the combined backpack GUI

Same issue here

To put backpacks in the combined backpack you combine it in the crafting grid. Then if you want, do /bm split to remove the backpack from the combined backpack.

thanks, that still doesn't solve the nested backpacks tho!

the backpacks.nest permission is what you are looking for, see plugin.yml