Reppro Step:
1-put your backpack(wearable backpack) down
2-put Item in it
3-Shift break it to put it back on your back
4-go to a armor stand (minecraft) and Shift-right click to put all equipment on the armor stand
5-all item in backpack fall on the ground
6-Shift-right click the armor stand again to get your backpack back
7-item are now in your inventory and in the backpack
8-STONK!!!

Likely going to remove that backpack mod in favor of Quark's backpack addon. It's much more vanilla friendly, I think, but has it's downsides. I'll keep this issue open until I switch to quark's. If I don't switch, Ill report this to the mod developer, unless you want to copy and paste this issue onto the wearable backpacks issues

removed wearable backpacks