Thanks for checking back. Nothing yet. I reconnected the primary device on Monday, it is still working normally, with the secondary device showing only the one (Device) tab.
It appears that with the 700 series HA, when the primary device comes back online, the secondary automatically resumes its backup role with the one device tab, correct? I expected when bringing the primary online, the secondary would continue its full GUI and the primary would show only the Device tab, until the secondary was rebooted.
This behavior appears to be different from the HA clusters (R77 and R80.3) I have created.