I've had this happen a few times, this usually corrects it:
Remove the service in the arcgis.com map.
Logout of the arcgis.com site.
Log in to the arcgis.com site.
Open the map where the service was removed.
Add the service back into the map.
Save the map.
On the iOS device, open a different map, allow that map to fully open, then open the map that had the issue. I suspect that this forces the app to re-read the JSON from the arcgis.com site.
Hopefully the issue is gone...?