Behavior of Box Selection when Extend or Subtract selection with Middle mouse.
PROBLEM:
After you Shift/Ctrl and Click to Extend/Subtract selection and after you release mouse it does not end the operation, you need to click again to end selection,
Expand the keymap item, there will be “Gesture Box” modal map (or you can find it at the end of the list) here you also need to change to the middle button, if I understand correctly what your problem is.