Some custom items lose theyr functionality if they get restored.
For example: https://rustplugins.net/index.php?/file/21-backpacks/ 
If player die and backpack custom item get restored - backpack no longer can be open anymore!
Not sure its restore folt or backpack it self...

I'm not sure about that Backpacks plugin, but Restore Upon Death has been tested and works well with Bag of Holding. Most likely there is an issue with that backpacks plugin. You could ask the author to test with Restore Upon Death.