I didn't find riseup-vpn in the 'ubuntu-software center', but could find it with snap search riseup.

After installing gnome-software-plugin-snap I could find it through the 'gnome-software'

I couldn't find the reason and I'm not sure what we should recommend people. Using Ubuntu bionic (dist-upgraded a couple of times) and having removed the configuration of software-center (cd ~/.config; rm -r software-center).

While running snapd in debug mode, SNAPD_DEBUG=1 SNAPD_DEBUG_HTTP=3 /usr/lib/snapd/snapd I don't even see requests/searches coming from the ubuntu software center.