I’ve had strange PixHawk behavior (on a clone PixHawk) before I brought in a separate power feed to my RFD900x. Let me amplify the warning.
Since the RFD900x can dynamically adjust transmit power, it can increase and decrease the amount of power that it is consuming. This will tend to happen as the range between TX and RX increases. It won’t happen when you’re testing on the ground with both the TX and RX turned on. So, this sort of failure can be fairly intermittent and insidious.
I believe that the one thing that saved me is that the RFD900x in the plane will use a lot of transmit power if the ground side radio is turned off, causing the brownout. If I had always powered my ground side radio first and quickly established a good link with the plane, I doubt that a brownout would have been seen while the plane was on the ground.
A separate power feed is important if you want to use the full transmit power of the RFD900x. If you do not have a separate power feed, I strongly suggest setting the RFD to a lower level of maximum power.