If the Curios Quark Backpack add-on is installed, then there is a bug with putting on two backpacks. Can you do something, if this add-on is installed, then the slot in the armor for the backpack is blocked? Only available for Curios.

https://legacy.curseforge.com/minecraft/mc-mods/curios-quark-oddities-backpack

In general, this function needs to be added to Quark itself, since no one needs a backpack in armor slots, it becomes useless. But in Curios, this is just what you need.