Hello,
my wife is having the same issue with her AW4 with Cellular for some time.
The difference between my AW3 with Cellular and hers were 700m on a 10km track - reproducible all the time.
I worked with several Apple support colleague approx 11/2 years ago and uploaded gigabytes of log file for them.
At a certain point they did not come back to me and I was tired talking to new folks. During my talks with them the issue got better to a difference of 200m only, but this is also not acceptable in my eyes.
As you all confirmed, with LTE turned off all is cool.
I also tested different service provider -> O2 vs Telekom and having the same issue.
I noticed that the Watch works fine in areas were you have strong mobile connectivity, but we are running in an area where you might have no reception or very low.
Anyway, last week I tested the AW5 with Cellular and this device is showing also the same problems.
Also did reset the watches several times and setup as new, did run with a phone doing a new calibration. Tested the internal App as well as Runtastic and Strava......
I'm at a point to sell the AW4 and buy a AW3 for my wife as turning off LTE is not an option, especially when she is running alone she wants to be able to place a call.
The only thing I haven't tested yet is the new Watch OS7 and someone asked if someone already tested, but till now no one answered. I still doubt that this is 100% fixing the issue, because I assume that this is a HW problem that the cellular module create interferences to the GPS module or something like this.
I'm really disappointed how Apple is handling this issue as you are also reading it in the official Apple forums.
Best regards