I've been using my fenix 6 solar pro for the past 8 days in expedition mode
where I've set the watch to continue to collect GPS points at night.
I've noticed that during the night the watch time drifted by 1-2 hours - every night.
Once I left the tent and got a GPS signal, the watch resync to the proper time again and remained in sync
all day long as long as I had GPS signal.
Those drifts are painful and made me calculate wrong my route plan.
I'm using the latest firmware that been pushed to the public.