Hey,
this is not proper mailing list for this kind of questions anymore given that wildfly does not contain web subsystem anymore.
in any case, my memory around this is fading so take this with grain of salt :-)
if instance-id is not set if defaults to
jboss.node.name in case you are proxying directly to web, but in case of mod-cluster integration mod cluster generates its own id that does not folow same convetion unless instance-id is defined.
I don't see any big reason why not change default, but it should in any case get bugzilla entry so it could be considered for fix in EAP.