Clef show courtesy off randomly shows courtesy clefs
(All instructions can also be done with a bass clef, and I assume, any clef)
1) In a piano grand staff, change the treble clef to an 8va treble clef.
2) In another measure (possibly only on another page?) change the treble clef back to a normal treble clef.
3) set the treble clef where it is changed back to normal NOT to show courtesy.
4) save the score.
Workaround: Click the courtesy shown clef after saving and set it as an invisible element.