Connecting stems causing side-affects
I'm trying to duplicate the connected stems in the attached picture; the last two sets of notes connect the stems from the two staffs.
I selected the note in the bottom staff, hit X to reverse the stem, select the stem, then drag it up to the line between the two notes above; so far so good. However, when I let go of the stem, the size of the gap between the two staffs is suddenly increased, leaving the stem now below the bottom note (the second attached picture is just after release; before I released, the stem was up to the line between the notes, so the gap has been increased by that much). If I ignore that, drag the stem again to the line, then move on to the note to the right and repeat the steps on it, the gap is increased again.
I obviously don't want the gap between the two staffs to be increased. Why is it happening?