In reply to Sharp:
I regularly run and ride through Forested areas, predominantly large Beech pollards but definitely mixed woodland of varying thickness.
I don't have a problem with my phone (Samsung S9) wich provides a really accurate track when mountain biking, this has definitely improved as I have got newer phones.
Running I use a Garmin Forerunner 10, so pretty ancient, this doesn't give as an accurate track as my phone, it is still spot on with speeds and distance but the polling rate isn't the same and the points aren't always within 5 metre accurate which I would expect from my phone, but this evens out over the length of a run. I would imagine a newer watch would provide more regular polling and a more sensitive GPS antenna. It certainly is nowhere near a problem that stops me using it.
My phone and Garmin Edge 520 that I use on the road bike both have access to Galileo satellites too which help accuracy, something to look for in a device.