Every time I place the backpack on the ground, put items in it while in my hand, or swapping its position from my chest armor slot to my hotbar, the backpack "recrafts" itself. I'll explain further: I get attacked by something with it on my chest slot, it obviously takes damage, and whenever I place it down and grab it up again it hasn't got any more damage. I said "recrafts" because I have the "You Will Die" mod installed -amongst other things, it also gives a random bonus in protection to any crafted armor piece and then changes its name accordingly. Well, if I at first had -let's say- a damaged "Old Cow Backpack" with an armor bonus of 0.5%, using any of the procedures above I could end up with a pristine "Hardened Cow Backpack" with an armor bonus of 3%, just as if I crafted another cow backpack for free. I don't know if that's intentional, but I suppose not.

NOTE: I didn't try any other backpack than the cow one, as of the time of this writing.

Welp, backpacks shouldn't take any damage to begin with. I'll fix this along with making it so the backpack doesn't take the armor slot when equipped.

Will not fix - as it stands right now. It will be fixed upon the new slot implementation.