Putting this in a separate post so that I may hopefully mark it as a solution...
After some more experimenting I figured out that it is possible to cycle through the 4 available views (sector map, full screen, top / bottom bars only, full UI) with the Page Up / Page Down keys. It's still strange that "M" does not work, but since I can get around that bug by using Page Up / Page Down this can be considered solved. Interestingly, though, the "M" key works again after cycling.
Of course I could have read the manual, but I've played JA1 / DG many, many times and thought I'd know it all... well :p Putting this here so that it may hopefully help others with the same problem.
Post edited July 08, 2021 by Warrgarbl