Volker, your trace is perfect !! You don't have the "drunk man" syndrome. What are your settings on the watch for GPS ? Are you using GPS + Galileo ? I saw you use Version 3.21, is it a Beta version ? I am with version 3.10 but on Garmin connect, it is indicated that I am up to date.
Thanks
Tracked my trip by car to my holiday destination Fenix 5+, fw 3.21, gps only, 1 sec, Tempe Sensor and bt connection to Iphone: battery went down from 80 to 46% (34%) in 9 hrs 24 min (about 3,58%/hour)
Hier ist meine autofahren-Aktivität bei Garmin Connect. #beatyesterday https://connect.garmin.com/modern/activity/2850099562
Could be the speed and perhaps the lock on road settings turned on.