I believe I can answer this, so I’m going to give it a shot since @jmorris won’t be in for a bit.
The name used to connect to the cluster is mostly dependent on how the cluster itself is configured. The names (or IPs) in your connection string are only used once, for initial bootstrap to a single node. The node map is then downloaded from the first node, and then the server names (or IPs) in the node map are used going forward.
I suspect what you’ll find if you examine your cluster is that, on the Servers tab, your servers are listed by IP. If you want to use DNS name validation, I would recommend that they be registered by their canonical domain name when the cluster is built.
Another useful tool is the SDK Doctor. It can help you diagnose connection problems to your cluster. I’m not certain, but I suspect it may give you a warning about this scenario.