I have a single device that is missing from the Devices list in the mobile app. System is running HS4. All other devices seem to be present.
Note that the device is an Insteon device Type "I/O Linc" linked thru the the plugin. Other I/O Linc devices are present. I've tried cycling the "Hide from Mobiles Views" setting, and I've change the location (room/floor) settings with no luck. The device has no comm failures according to the plugin stats.

HS4 Pro Edition (Windows)