This is indeed a change from SDK2 to SDK3; there is a resolved ticket and a patch in review for release in 3.2.1.
Once 3.2.1 is released, you can change the current behavior (which should resolve your issue) by doing this:
var options = new ClusterOptions
{
ForceIpAsTargetHost = false
};
var cluster = await Cluster.ConnectAsync("https://localhost", options);
Jeff