That was exactly my experience. I discovered when I yaw right, the aircraft would descend. Yaw left and the aircraft would climb. I went back through my remote files which indicated the problem started after the last firmware update. I made a spreadsheet of the flights with the values from the throttle channel which clearly showed the change in channel setting occurred following the update.
Yuneec refused to acknowledge anything. Period. They insisted I pressed B1. They said my spreadsheet was useless. After spending nearly 30 hours going thru telemetry files and tracing down the problem they spend 5 minutes to look at their proprietary software which shows B1 was pressed. Of course it shows B1 is pressed because that's the way the software is written. It cannot detect an anomaly. And the techs at Yuneec cannot read telemetry files. They are clueless beyond what their software tells them.
This is a rare anomaly and has only been documented a few times.
@RGilmore, who has extensive experience with copters, had the same thing happen, but was able to discover it before his machine crashed.
@Brian Mackey lost his aircraft with the same problem. The owner of Drone Mountain, a Yuneec dealer, had his motors shut down in flight without touching B1. I'm certain there are many that we don't know about.
