New worksheet in a long-existing (and working) file changes expression to string after saving and closing (and then reopening)

[Ron G <ronmlgaw yahoo com>]

I have a gnumeric spreadsheet with about 30 worksheets, many using the 
dsum expression.  However, a newly created worksheet is exhibiting 
unusual behavior (everything else but the new worksheet works normally) 
: it works fine when the file is open, but when I save the file, close 
it, and then re-open it, all the dsum expressions in only that new 
worksheet are converted to a string... e.g. =dsum(expression) becomes 
'=dsum(expression)

I can do a "find/replace" using the advance button to shut of 'Keep 
strings as string', replacing = with =, and limiting it to the range 
within the sheet (probably could just let it replace all in the current 
worksheet just as easily).

I'm using Windows version 10.16 (I know, ancient, but... I just can move 
so much faster in Gnumeric than Excel or Libre Calc, and haven't been 
able to resolve the compile errors when trying to build it myself in MSYS2).

Regards, RG