The Fenix 6 series is known for its poor GPS accuracy.
For navigating, the GPS accuracy is sufficient in my eyes, but when running, the poor GPS is a disaster, especially when you run in places where the GPS is disturbed, for example, under trees, in narrow streets with high houses at the roadside.
The new EPIX Gen 2 and - as far as the GPS is concerned - supposedly identical Fenix 7 does a much better job than the Fenix 6.
As you can see on the following graph, the accuracy of the track is much better, the power status drops much less under trees.
The instant pace, which is used for the performance status and at the end for the VO2Max calculation, also differs greatly between the watches, but is already corrected in the graphic in Garmin Connect. I have the instant pace displayed on both watches while running and while the display fluctuates greatly on the Fenix 6X as soon as you run under a tree, the values are much more constant on the EPIX2.