I have had the Fenix 3, 5x+, 6xPro and now the Fenix 8 51mm. Up until the 8, they have consistently reported about 2-4% shorter distance than what I believe is accurate when in tight switchbacks with tree canopy. I bought the Stryd foot-pod and get remarkable results with that.
Having said that, I think the Fenix 8 did fantastic on my first trail. It was just a hike instead of a trail run so this is not apples to apples but I think it may have even reported 2% more distance. I only have 1 trail since getting it but am really curious if it's slightly over-calculating. I am using straight-up default settings. I'm not by any means saying it's inaccurate as I don't enough hikes/runs for that but am very curious if anyone has done trail runs and their impression on the GPS distance accuracy. The tracks look great.