This is something that we are working towards improving with disabling sync on hosted feature layers, we are planning on it coming into the next update to ArcGIS Online.
Sync cannot be disabled on a feature service. Once it is enabled and the item is saved the feature service is updated with sync enabled and cannot be disabled. It is forced to enabled because the replicas you create from the feature service will not be able to be synced back if it is disabled and could orphan data. It has always been like this on hosted and ArcGIS Server feature services.
I think, for now, your option is to republish the service with Sync Disabled.
This is something that we are working towards improving with disabling sync on hosted feature layers, we are planning on it coming into the next update to ArcGIS Online.
Sorry to tag on the end of a postrussell_roberts-esristaff and jayanta.poddar. Just to check, does this mean I would be able to turn off sync on a layer, overwrite that web layers contents and then re-enable sync? And what would that do to people with content downloaded for offline use in Collector?
If you disable sync on the layer and overwrite the service you will orphan offline replica. When you have replicas on the service and you go to disable it through the item details you will get a warning about the risks of disabling and that you will risk losing the connection to offline data. (note all this is planned for the upcoming release and is not there today)
So will that orphan just that layer, or will it also orphan other layers contained within the same downloaded map? If its the former that's not an issue - the layer in question is a data dump of customer info we refresh once a week that would be useful for operatives to have access to. However they don't edit the layer in anyway, its purely for information purposes - its the other layers in their offline map they are editing on Collector. And don't worry I get that this is planned for the upcoming release - are you able to say when the release will be?
I seem to be having issues with this. The page is in a constant state of 'saving' my action to not allow sync and I can't get it to save. Due to this I cannot add my new time field that is necessary.
