An update on this to those who experience similar problem. I now 100% tracked it down to a bug in Asobo ATR 72. The aircraft battery does not charge even when connected to ground power. After few minutes the battery discharges and radios are lost. There are 2 workarounds:
- After loading aircraft, start the engine 2 in H mode OR
- If you battery got discharged, enable Developer mode. Then go to Tools → Behaviors → Systems → Show electrical system debug. In the top left, select Debug → Enable components edition. Set battery charge to 100%.