I have AOD set as always on during Activity . Generally it has worked as expected (as far as I can recall).
Yesterday I was scouting a local half marathon route using Navigation, Pace Pro and Live Tracking to test them.
For some reason AOD decided it did not want to play along so the screen was black until gesture - It was cloudy and under treelined paths so the AOD would have easily been visible. I stopped looked on the GC app , toggled the AOD during activity on and off, but it still decided to remain AWOL.
I will see if I can replicate. Did note as Beta Bug in the meantime, as I am on 4.15