Ok so I believe this issue is now resolved - at least the emails no longer bounce.
There's an important step missing from the Invision Community guide: you need to list the source domain, subdomain and emails you use to send emails to privaterelay.appleid.com in the Services tab of the Certificates, Identifiers & Profiles, or Apple won't recognize the emails and your notifications will bounce.
The Invision Community guide should be updated to add that crucial information and ideally with updated screenshots to match the new UI of the Apple Developer portal.
Go to the Services tab of the Certificates, Identifies & Profiles:
Enter any domain, subdomain and email sources used for email communication with Apple's private relay service: