In the latest 3.1.3 version an explosion that kills the player causes both the backpack to place a block containing its inventory, as well as drop the backpack and all of its content as items. Effectively doubling the backpack inventory.

I tested this on both 3.1.3 and 3.1.2, clean forge install with only Wearable Backpacks installed. Could not get bug to occur on 3.1.2.

Confirmed once more by #111.

I can also confirm this; just tested it on 3.1.3 and 3.1.2, and it is as the OP said. I'll be using 3.1.2 for now.