5 Plus - GPS Distance coming up as much a 20% short in the woods.

Is anyone else having GPS distance measurement problems with the 5 Plus in the woods? I'm in the northeast US, and running mostly technical trails under tree cover.
I've tried 1sec recording and Smart recording. I've waited as much as 5 minutes after the watch acquires gps to let it get locked in. I've tried GPS only and GPS + Glonass. The watch is regularly measuring close to 20% short on distance, particularly on twisty trails. These are courses I've run dozens of times with my last watch as well as having done plenty of group runs with friends who are using other garmin devices.

The point is, I'm 100% certain that the 5 Plus is under measuring the distance. I'm actually on my second unit, (thanks to Clever Training and they're awesome customer service!) and still having the same problem. I'm kind of shocked to not be seeing others having the same problem here on this forum.

Has anyone else had this experience or heard about other 5Plus users who have? I could really use some help here...