Hey guys, does anyone actually have an answer to the original question? I am experiencing this exact problem on Sky Factory 3 (doesn't use Inventory Tweaks, I don't think). Shift double-click will move all of one item to/from the inventory 90% of the time, but 10% of the time it drops all the items on the ground. Sky Factory 3 starts you in a void world so having chests near the edge of a platform poses a risk of dropping everything off the edge, and is incredibly frustrating.
Is this a glitch? Is it some keybinding conflict? (I can't find any key bindings for this)
Any response greatly appreciated