Comment 7 for bug 1574777

Mir does have some code that will throttle proportionally to the advertised refresh rate. That's the 'predictive bypass' optimization. Although even with that you should see a frames every second or two -- something would be on screen. So the monitor reporting (or Mir detecting?) an incorrect frame rate is not the main problem.

As shown in comment #6, Mir did detect the external monitor but it was only connected for 26 seconds. The log appears to show Mir notifying Unity8 of this successfully so I'm not sure who is to blame for the image remaining black.

One thing you can test right now is the cable itself. The biggest problem we have with external monitors on the phablet is faulty display cables. They are very common. So if you can, please test the cable on a different device, or test a different cable.