Finally stumbled unto what appears to be the cause of this problem.
It seems that the troublesome files were ones where the GIMP editing involved creating some extra layers which adds an alpha channel, which is something I really don't know much about. However, when you export such an image the .tif format supports inclusion of the alpha channel, which now appears to be what was causing the problem.
When exporting in .tif format there is an option called "Save Layers". It seemed to this novice that disabling this meant just give me an ordinary single layer image which is what was desired. However, doing that is NOT enough as it seems an alpha channel gets preserved. By first using the "Flatten Image" option in the layer dialogue the alpha channel/s get removed and the problem that provoked this post goes away. This makes it look like this is what Rawtherapee was doing when reexporting the problematic file.
This method of operation presents a slight usability problem that requires the exercise of some care. If, like me, the motive for retaining an .xcf file is to preserve the layers it becomes necessary to "Flatten" before export but then "Undo" the flatten or possibly exit without saving in order to retain the desired multi-layer file.
It seems that the troublesome files were ones where the GIMP editing involved creating some extra layers which adds an alpha channel, which is something I really don't know much about. However, when you export such an image the .tif format supports inclusion of the alpha channel, which now appears to be what was causing the problem.
When exporting in .tif format there is an option called "Save Layers". It seemed to this novice that disabling this meant just give me an ordinary single layer image which is what was desired. However, doing that is NOT enough as it seems an alpha channel gets preserved. By first using the "Flatten Image" option in the layer dialogue the alpha channel/s get removed and the problem that provoked this post goes away. This makes it look like this is what Rawtherapee was doing when reexporting the problematic file.
This method of operation presents a slight usability problem that requires the exercise of some care. If, like me, the motive for retaining an .xcf file is to preserve the layers it becomes necessary to "Flatten" before export but then "Undo" the flatten or possibly exit without saving in order to retain the desired multi-layer file.