It looks like the way it should be — first try the current fonset, only then fallback to ‘fontset-default’.

> And I still don't understand why you need that.  AFAICS, Emacs
> explicitly tries to use the default font for punctuation and symbol
> characters before falling back on looking up other fonts.

Do you mean that you could not reproduce this?

I’ve installed the lastest upstream version of Terminus [0] — and got the same issue.  So it does not seem to be Debian-specific.

[0] https://sourceforge.net/projects/terminus-font/files/terminus-font-4.40/