Thank you. I apologize that I doubt you.
If you installed Google components first, and initiated Signal installation afterwards, than I get your frustration. One last question do you use stable release of Play services or beta?
Maybe bug report would be helpful for the GOS developers...
In the meantime - if anyone wouldn't come up with anything better - I would suggest trying - but I do not if that would work - to install apk from signal.org/android/apk and when an update would be available to try install it from Play store. You would get notifications either way.
The second thing to try: if you have a spare phone, install and register Signal there. Create a back up. Uninstall Signal from GOS and install it again. Then initiate backup.