I am not acquainted with Addendum MP, and I have a feeling that maybe you should have included the appropriate mem-
tag to target people who know SCOM.
However, I have an idea of what might be the reason. On that server, open SQL Server Configuration Manager. Find the node SQL Server protocols. Right-click on the server itself in the left pane (and not any of the protocols to the right) and select Properties. Could it be that Hide Instance
is set to Yes?
See screenshot below.