Why no global position?

I have been getting a consistent “no global position” message on almost every flight with PX4 v1.8.2. The attached flight log will show two instances at different times during the flight. The first one happened for a very short time and the mission restarted. The second one was fatal and caused the DESCEND fail safe mode.
I’m suspicious of the continuous WARNING messages about txbuf allocation. Can anyone shed light on why this is happening?