Courier keeps reappearing as defaults font

Hi - I’m running Scrivener for Windows, and have set my default font as Calibri using Tools - Options - Editor, etc.

For a while this seemed to have worked, but now Courier has reasserted itself as the default font when I create new documents in my current project (wasn’t doing this last week!). In the Default Main Text Attributes box it says that my defult font is Calibri, but it ain’t so.

Can anyone help?

Many thanks.