Somehow it just isn't working with the default config file. Neither can i equip the backpack in chest slot nor does it display any defense values on it. I tried cheated and crafted backpack as well.

What happens if you sneak and break it? Does it show a message in the chat? Does it refuse to break? Does it not get equipped when broken?

Are you using a mod to display defense values? I think the reason why they're missing is due to the backpack not being actual armor, but using Forge's ISpecialArmor to act like it. What's that mod called again? Maybe it's possible to add support for it!

sneak/break: collected like a normal item in free inventory slot. No chat message displayed

Just tried without any other mods and still the same. I guess either a general problem in 1.12.2 or with newer forge versions (using 14.23.0.2501)

Haven't had the chance to reproduce this yet but I will certainly look into it and fix it for the next version, I think. That might take some time still.

Don't ask me why but somehow it's working now (maybe because i updated Forge to 2512 ?)

I was actually wondering if it was due to a change in Forge code. Looks like it was even a bug! Yay for me not having to look into it!