Interesting, I’ve had similar problems with the WIU+app and I also have a UniFi Wireless access point. Mine is a an AC Pro.
Very interesting. My UniFi AC Pro is located about 20 feet from MTH wifi unit but the signal must pass through a wall between the interior of the house and the garage where my layout is located. For years the system worked fine, but about 4 months ago I started to have connection problems with apple smart devices.