So, Phone restarted and downgraded to the non-beta version 7.7.6 from the Play Store.

Image 1: Connected via USB to Ford Sync 3 for Android Auto functionality - no speed data, but correct gps track and height information. The first spike is before I plugged the USB cable in.

Image 2: Same route reverse but not connected via USB to Android Auto. Screen shows correct information.

There is appr. 380GB free space on the phone, which should be enough. GPS tracking was working fine until yesterday when I switched cars and used Android Auto for the first time. Phone is a Galaxy S10+. If I zoom on the map the GPS track matches perfectly... it*s just the speed missing.
Will try again tomorrow on a longer trip.