Mac OSX 10.15.3, on a MacBook Air
I have 2 screens. Built in and Cinema Display (It's stacked above).
When Illustrator is expanded to full screen on the Cinema Display, the fx button doesn't show it's menu option. If the application window is less than the full height of the screen, it shows up. Note: My Appearance panel was at the bottom of the screen, so the fx button is at the bottom of the display.