Further Updates.
We have done some more testing
1. We have an issue with Vmware Data Protection and out of curiosity we uninstalled VDP and tested again to rule this out.. - No change issue still persists.
2. when the VCenter service is restarted the View Administrator Dashboard reports that Vcenter is down and when you click on it it reports that "The service is not working properly."
However Log off refresh and Pool recompose works as expected. If we wait for the View admin dashboard to go all green then do a Pool Refresh, Pool Rebalance or increase the number of a pool the problem is still there.
3. Recomposing the pool to an Earlier Snapshot that was problematic works fine.
All in all from my investigations it seems that every time Vcenter service restarts it seems to corrupt some of the View Replica machines. Forcing the system to create a new replica and delete the old Replica can stop the issue happening but any other Vcenter Service restart event just creates the problem again.
Vcenter service restarts can happen quite often. any Reboot of the Vcenter server after updates and HA events for example.