The fix seems to be trivial. I changed the arguments passed to gunicorn3 to be as recommended on the official Isso docs for gunicorn here:
https://posativ.org/isso/docs/extras/deployment/#gunicorn Specifically, I changed the ExecStart line in isso.service to the following: ExecStart=/usr/bin/gunicorn3 -n gunicorn-isso -b localhost:8080 -w 4 --preload --log-file /var/log/isso/isso.log isso.run (I've no idea why Debian ships without the -b, -w and --preload args, or why it's invoking isso.dispatch instead of isso.run)
