Strange one in Exchange 2016. A Mailbox server was running for a while at a customer, tried to add the server to a DAG but it just failed.
Decided to check a few things and on the Exchange Admin Center (EAC) I noticed it said no roles on the server in question. Launched the Exchange Management Shell and ran the following command:
- Get-ExchangeServer -Identity <ServerName>
Below also showed that the server had no roles installed:
Very very strange indeed, launched the setup from an elevated prompt and installed the mailbox role again without an issue, after that the ServerRole info updated to Mailbox:
Hope it helps.