This is the expected behavior as it appears that any user accounts with a "Common Area Phone" license applied to them cannot be added to any contact groups in the Teams client.
As you've discovered you can search for them:
But attempts to add them to a contacts list will produce the error "We couldn't add a buddy to the group":
This seems to be a limitation placed on the lower-cost CAP licenses. If you need this capability you could use the more expensive Meeting Room Standard license or an E1 license on the accounts, yet still configure them with Common Area Phone behavior as covered in this article: