When the OP stated he was on top of a garage, I should have realised he was talking about a multi-storey car park - international structural term differences! Yep, that is a slightly daft place to T/O. Also, you can get terrible wind votex on the sides of these structures, so that could be a factor too, together with the increased wind speed 100' in then air at T/O and updrafts form other buildings in the vicinity. There are so many potential causes of the loss of the AC, let alone the compass issue which, of course, the most likely. One other thing; even if dji did notice an issue with the AC in the telemetry then they could state given you were not using dji authorised flight software, your warranty claim would be void.