The only time I've seen the hosting server list is greyed out is when the certificate isn't trusted.
Portal makes a request for the data store items via the Portal's internal proxy. If Portal doesn't trust the certificate, then the request fails and the drop down is greyed out, as it can't get a response that indicates the federated Server has a registered relational Data Store.
The network traffic will capture the 403, indicating that the URL is not trusted, and the Portal logs will record an error about the certification path not being trusted.
The certificate itself could be valid, sure. I typically use a certificate issued from our domain certificate authority. It's 100% valid, but the Portal will never trust that certificate by default because it's not in the trust store. If you use the self-signed certificate, then it's trusted by Portal by default.
If you see this, check the network traffic, and you see that a) the request for the data store items is a 200 and the request body contains the information for the registered relational ArcGIS Data Store, then yes, I could see this as a bug.