I’ve just updated Elements to V3.5.11.1054, on Windows, and a score that I had been working on in the previous version (1020) has missing note heads and other stuff when it is loaded and converted - it looks very barren indeed. Is there something I need to do related to fonts?
Have you restarted your computer?
That fixed it - thankyou! 
The install didn’t request a restart - should it have?
I’m not entirely sure under what circumstances the installer requests a restart. Some Dorico updaters/installers install new or revised fonts, and some don’t.