I'm wondering what the Replicas resource (http://<featureservice-url>/replicas) in the REST API is for? I was assuming it would show all the replicas generated by the ios SDK to support offline editing, but it never seems to have anything in it? Am I misunderstanding this or is there something wrong with our instance of AGS?
It should contain the list of registered offline geodatabases with the feature service. Check out the sample service for an example - http://sampleserver6.arcgisonline.com/arcgis/rest/services/Sync/WildfireSync/FeatureServer/replicas
When you sync your edits, it will check to make sure what you are trying to sync is registered with the service, otherwise will likely throw an error saying it can't find the replica GUID. Not sure why you aren't seeing anything on your service - maybe your apps are unregistering through the sync task?
We just updated to 10.3 last week and I hadn't gone to look to see if replicas were showing up in the list until I saw your post and it looks like its working now. Perhaps it was a bug at 10.2.2 or just something wonky with our install. Either way, I'm happy to forget about this issue and move on!