Using the Hephaestus Piggybackpack while wearing a Cammie's Wearable Backpack backpack, will delete all the contents of the backpack, and put the backpack on your inventory.

Just happened to me as well. Came here to see if it had been reported.

Marking as complete, it isn't fixed but the recipe for piggyback is temporarily removed in 2.0.4c and we'll be switching to a new backpack mod (travellers) in v2.1, which has proven to be way less likely to bug out like this!