It seems that the Font Face command does not actually fire, or at least does not insert its span with a font-family style, into the Content area of the RAD Editor until you actually start typing. Therefore, if you select a font, then immediately use another command, such as Bold, the selected font is "forgotten". When you start typing, you will see that a <strong></strong> tag has been inserted, but the "font span" that should have been inserted when you selected a font is missing.
To reproduce the problem:
Select a font
Use any other control, such as Font Size or Bold
Notice the Font You chose has been reset
Notice you are not typing in the font you selected.
Switch to HTML view, and notice that there is no <span style="font-family"> tag
I have tried injecting that "font span" immediately into the content area during the "OnClientCommandExecuting" event, when the command is "FontName". Unfortunately, while this does insert the font span, it also puts the cursor AFTER the injected span, so when the user executes another command, such as Bold, the font selection still appears not to be effective. To remedy THAT problem, I attempted to get a Range object from the current selection, manipulate its start/end, and then select it. However, I could not make that strategy work reliably.