Which file is open when return to Excel in Office 2019 Mac

[RSP123]

My issue is that, when I leave Excel to go to another application and then return to Excel, the last spreadsheet that I was using prior to leaving Excel, is not the file that appears on top.  A little experimentation shows that the file that appears in this case is the 2nd to last file that was open rather than the last file.

I can't find any documentation, but it appears that a checkmark in the list of open files designates the file that is on top, while a dot indicates that the file has unsaved changes.  In the above scenario when I go off to another application and then return, the checkmark is on the last file on top when I leave but is on the 2nd to last file when I return to Excel.

Does anyone have any suggestions as to why this is happening and how to fix it.  TIA.

[#2]

I thought I would update this thread with what I think was going on.  It appears that I had some files that were full screen, while the spreadsheet that I had left to go to another application was not.  When I returned from the other application, the full screen file appears rather than the last file in use.  It was simply a coincidence that the full screen file happened to be the second to last file opened, resulting in my incorrect hypothesis.