Are you sure the layer loaded OK? I would have expected a load error in this case.
I can't explain why ArcGIS and an Enterprise server would have failed at the same time except for some network/firewall issue, possibly exacerbated by caching on the device. Did you test on more than one device, and were you behind a firewall/load balancer at the time that might have blocked traffic or cached a bad response?
If you see it happen again, you should check the loadError property on the AGSServiceFeatureTable, and can use AGSRequestConfiguration.debugLogRequests and debugLogResponses to log network traffic to the console. This should tell you what's going wrong. You can also control the URLSession cache policy with the requestCachePolicy property.
You can either control the global AGSRequestConfiguration, or copy() the global one, modify the copy, and set the copy on the AGSServiceFeatureTable.requestConfiguration property to just control calls to that service. This is true of any class that implements AGSRemoteResource.