Why no global position?

I have been getting a consistent ā€œno global positionā€ 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.
Iā€™m suspicious of the continuous WARNING messages about txbuf allocation. Can anyone shed light on why this is happening?

Thanks,
Joe

https://review.px4.io/plot_app?log=d98b464e-7de5-41e4-812f-68ba8454ef27