In 4.8p3, I just discovered that the save button in the top menu bar does not prompt for a filename after a new map has been created.
The map is saved automatically with its default name “Untitledx”.
So, to reproduce:
-Create a blank new map
-Click on the Save button on the top menu bar
*You will now see that the map is saved, but with no option to choose location or filename.
If the “Save All” command is used from the content browser menu, the filename prompt opens and one is able to name the map and sace it to a specific folder.