A very annoying behaviour is that Excel opens an Excel file on the virtual Desktop of an existing Excel Instance and switches to that Desktop.
If you want to avoid this behaviour you can do the following to fix it:
The fix is to call Excel with the /X parameter:
$ assoc .xlsx
.xlsx=Excel.Sheet.12
$ ftype Excel.Sheet.12
Excel.Sheet.12="C:\Program Files\Microsoft Office\Root\Office16\EXCEL.EXE" "%1"
$ ftype Excel.Sheet.12="C:\Program Files\Microsoft Office\Root\Office16\EXCEL.EXE" /X "%1"
Excel.Sheet.12="C:\Program Files\Microsoft Office\Root\Office16\EXCEL.EXE" /X "%1"
I don't use Word very often but for good measure I've checked if Word has the same annoying behaviour but it looks like the default call syntax includes already the /N parameter.
$ ftype Word.Document.12
Word.Document.12="C:\Program Files\Microsoft Office\Root\Office16\WINWORD.EXE" /n "%1" /o "%u"