The clef popover accepts “mezzo” and “mezzosoprano” for mezzo-soprano clef, but “mezzo-soprano” with a hyphen immediately crashes to desktop. (Running Dorico 4 on macOS Monterey 12.3.1, M1 processor).
I can confirm it also happens on Windows 10.
I’m about 99% sure I’ve seen this reported and acknowledged by the development team already, but I can’t find it just now - sorry!
Also happens in Dorico 4 on my i7 iMac running MacOS 11.6.6.