Icon "save" does not save changes if used in connection with a png-file in some cases

there is given: a png-File created by any software

• right click it
• choose 'open with', choose inkscape
• add by 'copy and paste' further graphics around the existing one
• enlargen the canvas or not
• click the icon "save"

nothing happends, the changed object is not changed. The png is unchanged.

• use "save as.."

=> this works as it should, the save dialog for *.svg appears.

What is expected? The save icon shall work as the "save as.." does by the default settings.

(Inkscape 1.1.)