Is this the explanation as to why the transponder of the Legacy dropped offline?
Report: LOT E170 near Varna on Jun 30th 2015, transponder inadvertently off results in near collision with business jet
The Transponder of an Embraer ERJ-170 dropped offline over Europe causing another near collision. The Bulgarian AAIU describes the failure mode of the transponder in detail leading to the transponder dropping into standby mode while in flight ...