@Kjell-Rilbe From a casual observation.
Important to note: The Save As dialog has a checkbox named Append extension.
If Append extension checkbox is checked, then the filter combobox named Save as type will auto-select the filetype to match the existing file type or language. So if Save as type is Normal text file (*.txt), then it is known for Notepad++ to append .txt to the File name in edit control on save.
If Append extension checkbox is unchecked, then the filter combobox named Save as type will auto-select All types (*.*) as the state of appending a file extension is undetermined so that the user needs to type into the whole file name including file extension to save as a file type.
Perhaps for the 10+ years mentioned the checkbox Append extension has been unchecked for your Notepad++ configuration. One day it changed and the usual routine afterwards was disrupted.
Just viewed config.xml with changing the checkbox state and closing Notepad++. The state is remembered for the next Notepad++ instance:
<GUIConfig name="MISC" ... saveDlgExtFilterToAllTypes="yes" ... />The value can toggle with value yes and no. There should be no need to change this value manually if a Gui method is available … ( Append extension ).
The Preferences dialog shows no mention that I could see to influence the state of the Save as type combobox and so I consider the Append extension is the main point of change.
The suggested fix for you is to uncheck Append extension on the next Save As event.