It took me a little while to pin down death as the reason for it resetting - for what it is worth, the backpack does drop when I die in my setup, and I'm noticing the loss in progress when I put it back on again after retrieving it.

Thanks. I'll get a fix out asap. If you were close to unlocking a backpack, you can use the command /unlockbackpack

Should work fine now in 2.0.3