Signal has been discussing related issue on discovery [1,2] for a while. However this research shows this issue in a new light, and I'd be happy to see a better resolution.

[1] https://github.com/signalapp/Signal-Android/issues/6750

[2] https://community.signalusers.org/t/why-are-users-who-have-m...

Additionally for Signal users: It is possible to turn the notification feature off, but if you newly join Signal, every Signal user in your address book will be notified unless they have switched it off.