Does Strava suffer accuracy when used in parallel on iPhone (X) and Apple Watch (series 3)?

Hi,

I am getting a bit confused about the (GPS) performance of my new equipment (iPhone X and Watch series 3 w. LTE). Problem is that lately Strava (activity tracking app I use for my MTB rides the last 3 years) logging is very inaccurate (completely off-track at times and absolutely missing parts of the track) whilst everything seems to be OK on both my watch and phone. I even purchased the GPS Diagnostics app to see whether this would reveal issues but got all "satellites" green when outside (at home they are orange and between 40 and 70%)

I was wondering whether the fact that I have both devices with me during an MTB ride could be the cause of this inaccuracy:

Last couple of months I only took my watch with me as my friends took their phones with them (I do have an LTE enabled watch but can not use it yet over here in the Netherlands). During those session (using the Strava Watch app which syncs its logged data with the strava server once I get home again in wifi network) all is OK and on par with what I used to see with my previous iPhone (6), I used for more than two years.

Last couple of rides however I was on my own and decided to take my phone with me as well ... I have the feeling that this is the moment that the inaccuracies started to occur: Completely skipping parts of the single tracks and missing more than 10% of the total distance (5+ kilometers on a 50km ride)!

I read somewhere the the watch will automatically start to use the GPS from it's paired phone (when nearby) which could explain that, in case the GPS accuracy of this phone is inaccurate, the results of the logging via my watch will be as inaccurate. To eliminate this uncertainty, I decided to start strava on both, my phone and watch, during a 3km walk this afternoon however disabling bluetooth (this way making sure that the watch can not "see" my phone and thus have to use its own GPS hardware. Guess what: Both sessions turned out to be very inaccurate (at different spots on the track), at a certain point "jumping" app. 500 meters, resulting in a total distance being app. 1.5 km off

It makes me feel a bit uncomfortable on what exactly is going on ... is it the hardware, is it a problem with Strava (also got very inaccurate logging when using the built-in activity app on my watch (however with my iPhone with me as well)) or is a problem with iOs/watchOS?

Anybody else in here having the same kind of experience lately with the device(s)' GPS? Any clue on what might be wrong? Any advice?

KR,

Jan.