If isolation of jboss instances is being done via port offsets using the port-offset attribute on the socket-binding-group, one still has to separately update the ports in the management-interfaces:
<native-interface interface="management" port="9999"/>
<http-interface interface="management" port="9990"/>
I'm not sure why this cannot be configured as any other port. At a minimum, the management-interfaces element should also have a port-offset attribute.