I don't think this is a bug. Iron Backpacks mod has it own upgrades including Button Upgrade which adds sort and transfer buttons.

Well, the Iron Backpacks upgrade doesn’t have the merging option. It’d be nice if Quark’s buttons appeared in the Iron Backpacks’ backpack GUIs or if there were a config option for it.

I guess. I'll see what I can do about it.

Thanks.