Do you have the Cheat Sheet installed? It looks up all the keyboard shortcuts in apps and display them when you long-press the command key. If Scrivener beeps, the shortcuts shouldn’t exist, or could possibly be mapped to something else, which should be seen in the Cheat Sheet.
This is a Catalina related bug for sure. And it seems to be limited to Font-Types that were manually imported by the User. Possibly due to the new System Files Structure in Catalina (?)
Everything worked fine until I upgraded to Catalina three days ago. Since then, I cannot use the Keyboard Shortcut to switch to Italic, when using font Candara. (Which I manually imported a long time ago…)
The funny thing is: I can switch to Italic if the text is already Bold. So I can use Bold, then Italic, then “Bold” again to undo it, and what’s left is purely Italic text.
This proves that the problem is not the Keyboard-Shortcut (e.g. being blocked by another App), because in such a case it also would not work on bold text.
However, when using a Apple Standard Font (like Palatino), I can switch to Italic easily.
I cannot say for sure, but the problem could be related to the file location of Fonts:
In Catalina, a “secure” partition was created for System Files. (Actually, your Hard Drive is now split in two, you just don’t see it.) The folder /system/library/fonts cannot be modified anymore. (I cannot even grant myself Admin-Rights to this location, to copy-paste the Fonts there manually.) Manually Imported fonts end up in a separate location. (…/user/library/fonts).
Maybe the problem is related to this separate file location? E.g. that Fonts are not loaded correctly in Catalina? Or are somehow unreachable by Scrivener?
Would be interesting to see what happens if a non-native Font can be copied directly into the System folder. I guess that it would work. But I fail to get the necessary permissions.