That's a good solution, a.l.e and, in fact, I'm about to take down a list of fonts to copy across to my sandbox. However, this is only a workaround and doesn't solve the intrinsic problem.
Your suggestion that this is a DM problem is an interesting one (I'm running Plasma on Chakra which, as I said, is an Arch variant). Why I posted this question here is that my other applications have their menu bar. LibreOffice Writer, for example, Gimp, Thunderbird, etc etc. So I'm wondering if there's an additional Scribus config file lurking around somewhere that's messing about with things. If this were a universal setting, I'd expect my other programs to be missing their menu bars too, but they're not. It only seems to affect Scribus. I thought if anyone would know the dark, arcane secrets of Scribus

it would be this group...
Kaz
UPDATE: I misspoke. It affects Kate as well. I may go further afield to see if I get any info and update here if I manage to solve this...