Nicolas,
What exact BT devices are you using?
In fact, i'm sure that it's the arc touch mouse which causes the problem, not all bluetooth devices. And this mouse is very useful when i'm doing powerpoint presentations (i'm a teacher).
Thanks
Nicolas,
There are reports of this issue with phones connected via Bluetooth in this thread. Do you have a specific BT device that you can connect without this issue? It appears that if you connect a device AFTER negotiating the Miracast connection that the issue does not occur and the res stays at 1920 x 1080. Can you confirm this? Start the Miracast session with the mouse flattened and inactive and then wake the mouse.