We got it working. Details here for reference.
To recap, 3 clusters running in an ESX environment using NSX-T for the networking.
Each cluster configured as a separate cloning group so the configurations match.
On 2 of the 3 clusters, when we enable OSPF on a single interface, the standby cluster member fails with a ROUTED PNOTE.
The 'fix' was to break the cloning groups, reboot each member, reconfigure OSPF on each individual box, then enable the cloning group again. Now all 3 clusters are happy, although somehow the OSPF interface moved on one of the clusters