Well we just updated H9 Control yesterday to version 2.9.3. Did this just start after updating H9 Control, or was it occurring before? I guess I'm trying to understand if this is due to a small change we made in the latest version of the app or due to the changes in iOS 11.
Also, if I were you, I'd try the following. I'd reset the system settings on my H9s by restarting them with the right footswitch and the HotKnob switch pressed (the switch in the upper left hand corner of the H9). And I'd try resetting the network settings of my iOS device by going into the Settings app, going to General and going to Reset. If you wouldn't mind giving this a try, could you let us know if that helps?