if you are using the widget to override the light mode then whenever you turn the garmin off or put it too sleep at say a stop the default mode will be reinstated on power up when the light network is re-formed. You would have to change the mode in the light sensor settings to have your mode as the default
hi,
I had same issue on fenix 5X+.
but lately, I'm not able to pinpoint the moment, after some updates (many betas I'm now in 7.52 beta) it is stable. How I set it from the beginning of the ride, remains till the end...rides 10-40km long... aslo started in daylight and finished in the night.
robert