Can't connect to named instance on FCI without specifying port

Hey all,

I'm struggling with an odd problem on my new SQL cluster and wondered what I am missing. Briefly, I have deployed a 2 node cluster with two instances, one default and one named, on separate cluster network names.

I have servers SQL01 & SQL02
Virtual network names SQLCL01 & SQLCL02
SQLCL01 has a default instance on it
SQLCL02 has a named instance called Instance2

I can connect to the dafault instance all day long no problems, but the named instance is giving me some jip. From the SQL server connect dialog, I can browse and see the instance in the list. If I connect from our old SQL 2014 server (Management Studio 12), it connects immediately no problems whatsoever.

When I try to connect from a new server I have spun up (on the same subnet), or my desktop machine (different subnet) with the latest Management Studio (18 or 17) on it...