I have implemented Firebase Invite with custom email HTML and everything worked fine on my development phone when debugging with Android Studio.
I then built a release signed APK and sent it to my personal phone. Tried sending an invite (to the same addresses as before) and it didn't work - I didn't even get the popup message saying it was sent.
I uninstalled the app, connected my personal phone to Android Studio and ran a debug version of the the same code and voila, everything is working...
What is wrong in release build?
I solved it but it looks like a FireBase/Google bug:
I regenerated the SHA1 key in Firebase and suddenly Invites started working on release build. I think this is a bug because obviously the previous SHA1 key was valid because I generated it based on a release key that allowed me to publish the app in the Play Store etc... (so it definitely was not a debug key...).
I submitted a bug report but haven't heard back about it...