The only way I have been able to correct this is by opening a ssh to the ESX host and send the following commands:
service vmware-vpxa status (to check service status - should be running)
service vmware-vpxa stop (ok to pass this command while hosting vm guests)
service mgmt-vmware restart
service vmware-vpxa start
Then check status in Virtual Center after a couple minutes and you should get your status back. I am hoping this is fixed in the next revision.
It's very strange, but after doing a migrate to another host, they also came back on one of the hosts. The others came back shortly after. HP have confirmed that your answer would probably work as well. Thanks.