Engrave mode Staff Spacing issue

As I am making time signature changes in various places in a piece, the vertical spacing goes awry quite a lot as sections get pushed around. Then when I go back in to set the distance from the top again by hand, which I normally set to 20mm for the first system, some of the pages no longer show the measurement box and some do. Is this some sort of file corruption? What may cause this? The bad aspect of this is I can no longer enter an exact measurement on those pages where it is missing. Are we in bug territory?

engrave 1

engrave 2

If you select the red handle at the top and hit delete, does the spinbox come back?

Would it not make sense to alter the music frame margin in Layout Options, or the dimensions of the music frame on the page template/master page?

You sir, @pianoleo are a genius. That fixes it. How did you ever work that out or know it? Most grateful.

Also you have awakened my dim consciousness to the idea that I can set the frame size to pretty much do what I want without so much manual tweaking. Never thought about it!