OK it was too weird not to be something dumb: this happens when there is already a note with the same name.
In this example a note named “Bad Education” was already in my vault but the warning about duplicate was hidden by my custom CSS 
BUT I had already checked if it was a duplicate problem by searching for example filename:Bad Education which shown no result, although after checking directly in the File Explorer there is indeed a file in my vault named Bad Education.md.