Is this a controller setup? Are the buttons not customizable?

If not and this is a mouse and keyboard hotkey issue, I control click the stack to transfer things myself. Then here is no way that, for instance, an older mouse glitches up and "releases" the held down mouse button when click dragging an item stack. I've found that worn out mice often do that and it makes archery a real pain for a min until I get a new one.

Anyway, it might be worth it to try that method and learn something completely new so that you don't default to the old hotkey by mistake.