When I open my backpack with something in my offhand, I expect the backpack to have priority, and open without anything else happening.
However, when I do, the offhand item begins an action WHILE I'm in the backpack interface. This includes actions such as eating food, placing blocks, etc.

A SEPARATE problem (but probabaly related) is that the backpack will not open when used in the offhand with any item in mainhand.
If the mainhand item has no interactions (e.g. tilling, placing, eating), a sound and animation will play for interaction, but the backpack gui will not appear.

I have other mods installed but I don't think any would interfere with this interaction. That being said, if you need anything, lmk.

visual example of food in offhand, being gobbled when it shouldn't: