@Brenden If you're talking about the setting " Versioning & Synchronization > Web farm > Allow synchronization for: Smart search indexes" then this is already checked. As I mentioned, the indexes synchronize fine when all web farm servers are present. But they go out of sync when one server is taklen offline for some time because when that web farm server is re-registered its file system is not automatically updated with latest indexes.
@Jeroen Thanks for your advice. Yes I'm aware of this. Actually our configuration is considerably more complex than I've outlined (dozens of sites over a large web farm for a start) so right now I'm just keen to understand if the behaviour I've described is to be expected. I'm assuming scheduled tasks are not involved in the transitioning process of a new web farm server object.