Wow, I somehow think that attributing anything other than fall or lava or acid damage to a player is a really bad idea. Sure it makes it convenient that exhaustion and fall damage don't give us repair costs, but this seems like a shortcut that might cause other bugs in the future. Maybe reclassifying Mortar Volley as boss damage will fix the bug?
P.S: A big thank you to Methodical for catching this bug.
The bug has been fixed months ago, stop posting here.