It has been my experience with 6.40.2 on a network of ~30 Mikrotik routers, that in order for a dhcp-relay anywhere out in the network to work, the packets from the relay must be received by the dhcp server on the same interface the dhcp server is configured on.
Now consider the case of an OSPF ring where you cannot predict what interface the packets will arrive on. When a link failure occurs, OSPF reroutes everything over backup links and the interface over which the packets from the relay arrive, changes. This assumes you're running DHCP on a router that participates in OSPF and has multiple links.
Is there a way to make it work in this case?