I am working on a mod pack that includes chance cubes. If you die in a room created by a chance cube while wearing the backpack you loose it. We are also using the grave from open blocks. I set the config to not have the backpack become a block upon death. It becomes a loose entity instead of being pulled into the tombstone.

One thing that the grave from open blocks does is spawn a dirt block if the it has no block under it. You may be able to do something similar to help with this issue.