41.71, singleplayer, completely vanilla, new save.
I started a fresh custom game mode after updating to 41.71 with the starter kit enabled.
My school bag works just fine as a separate inventory section, but the duffle bag doesn't allow me to put anything into it or take anything out while equipped on my back.
Steps to reproduce:
Find a black duffle bag from a zombie in the wild.
Kill it and equip it on the player's back.
Also, I placed some items in it to see if before picking it up, but that doesn't help.