I updated my 265 to 20.26 beta today and went for a bike ride.
I did 20.7km's but the watch only counted the whole workout has 4.7m's, the path was correct and all the rest of data was.
I'm sorry to hear about the inaccurate distance. Please read through GPS Drift and Environmental Factor Impact on GPS Accuracy, and then try the following for your next activities.
If you are still experiencing GPS issues, I suggest contacting Product Support directly!