In Activities or in Events, only tracks, created at mobile app can be used. That's not fair, because tracking in mobile app has a big problem: It seems, the mobile app is trying to upload parts of the track high frequently into the cloud of MyRouteApp. But if internet connection is (temp.) disrupted, part of track will not be uploaded! As result, you will get a track with lot of breaks!
As example see: https://www.myrouteapp.com/en/social/track/132145?mode=share
It wasn't a GPS problem, because my GARMIN etrex 30 has tracked well in same time: https://www.myrouteapp.com/en/social/track/143598?mode=share
Also, if we have done a route plannig, it will often happen we can't or we won't follow exactly the route (partly). Sometimes, in such cases, we will find special places or streets, we want to remember and save and use later again. But if tracking was disturbed at this time, we don't have a documentation where we are.
That's really a problem of the mobile app and not of devices (hardware). Because if i use parallel several other navigation devices (for car or for hitchhiking, they doesn't have a problem in same time to do tracking.
So, my assumption is: mobile app has a problem in beeing constantly online.
May be, this will be not an easy fix and need lot of time to develop - may it will be then more practically to be able to upload gpx track files into Events or Activities on Web (created by any devcie).