The behavior is that the external_nodes command is called with progressive arguments in this order:
e633ac9e-ef87-45aa-952c-9c09c319e06e.usma1.compute.amazonaws.com
e633ac9e-ef87-45aa-952c-9c09c319e06e
default
Where e633ac9e-ef87-45aa-952c-9c09c319e06e is the certname I used.
It's hard to say that this is a bug. I can see that for normal operations it might be desirable to map hostname -> fqdn 'smartly', but I'd rather that was the second iteration rather than the first. I would prefer that on the first iteration it's the plain jane certname (CN) as presented by the client's certificate without any frills on it.
Either I would like the behavior changed for external_nodes or possibly in general (although I don't know if this breaks anything??)
I'm willing to write the patch if this sounds like an acceptable fix. Let me know.