Hi David,
I have the same problem and i don’t know what to do (it appear after upgrading to wordpress 4.8 )
I trashed the browser cache etc before and after the update and it’s still not working. I try in three browser (safari, chrome, firefox) on two different computer.
It’s not the only problem : I can’t use a specific font family in custom CSS since the update to wordpress 4.8
Thank you !