If a Google Play tester is added to the alpha and the beta channels, they will (correctly) see whatever version is newest. So they'll see the alpha build if there is one available and the beta one if not.
However, if they start as a beta tester and are added to a group in the alpha channel, it doesn't seem to work. They still only see beta channel versions no matter if they leave the test, uninstall, etc.
What needs to be reset or changed so that a previous 'beta only' user can correctly be in the alpha and beta channels?