Went for a bike ride this morning with updated F5X. Noticed the screen flashing to black with a message saying "Wheel Diameter Updated" or something equivalent, I have the old style speed/cadence device with magnets btw.
I have seen this before in the past but only once during a ride.
This time however it was every 30 seconds or so. Irritating after a while. I have now done a soft reset and will report back if the problem is not fixed on my next ride.