Incorrect calculation of distance when walking under trees

I specify that I run with a Garmin HRM4-Run Belt.
When I run under trees, the calculation of my instantaneous speed is wrong: it goes from 6 min / km to 12 min / km although I have not slowed down.
the calculation of the total distance of the route is also wrong: the distance calculated on a map is 7.16 km, since there are leaves on the trees, my route varies between 6.7 and 6.8 km!
Worse, he sometimes thinks that I stopped, even though I was constantly running!
Is it possible to configure the watch to correct these errors?

The effect is very visible on the race pace graph.