As part of setting this thing up I had to learn a bit about how a WSGI app and the path under which it is mounted or exposed in a server’s URL hierarchy interact. The main key was this page that describes the rather obscure SCRIPT_NAME variable and how it designates URL components that WSGI will chop off or add back when sending requests back and forth with the fronting proxy (Apache in this case).

Long story short, in the systemd unit that starts the gunicorn/flask app I had to set SCRIPT_NAME=miniblog and in the Apache proxypass config, this:

       RequestHeader set X-Forwarded-Proto "https"
        ProxyPass "/miniblog" "http://localhost:19891/miniblog/"
        ProxyPassReverse "/miniblog" "http://localhost:10891/miniblog/"