First of all it MIGHT BE only a thing on my own side for unknown reason, because it's beyond annoying so it's not possible that no one ever noticed it. So if you played a couple of modpacks with MT Unofficial but never ever noticed any problem like this, just close it.

When opening a GUI with slots (like chests, player's inventory or crates from other mods), there's a chance that items in the slot at the center of the screen (because when opening a GUI the cursor is forced at the center, so yes it's only triggered when there exists a slot) get pulled, or scrolled on, even though no physical scrolling is actually performed. I've made sure that the mouse is not malfunctioning, and with Scroll item scaling set to Relative to scroll amount it can transfer multiple items at once without even triggering the animation from NeverEnoughAnimations multiple times, so I guess it's game itself that's bugged.

It's worth mentioning that it often happens after scrolling through the hotbar then opening an inventory. So spamming E won't work. You have to run around and scroll the wheel a bit, then stop in front of a chest containing multiple stacks of items, and open it.

Can't seem to reproduce with the official, original Mouse Tweaks. No such issue.