"Save As" does not write old file name to the "Open Recent" list
When you do a "Save As", you create a new copy of the file and it's this copy that remains open in MuseScore. There's no longer an active edit session open on the old file. At this point I'd expect the old file name to appear in the "Open Recent" file list, but it's not there. When the copy of the file is closed, both the original name and the new name are added to the Recent list at the same time. This seems wrong to me. I'd think that as soon as you're no longer editing the old copy of the file, you'd want it to appear in the Recent list in case you want to open it again (say, to compare the changes you're making to the new copy with what was in the original).
I noticed this originally because MuseScore crashed during a File/Close that immediately followed a File/Save As. When I restarted MuseScore neither file name appeared in the Recent list, presumably because the crash occurred before the step in File/Close that updates this list. I thought at first that I was experiencing a rerun of my lost file experience of two weeks ago, but fortunately only the Recent list was affected; both files were just fine.