If you scroll down the activity list to the bottom and click the ‘Load More’ button, it seems to skip some. It appears to leave an approx. 24 hour gap from the last activity shown, to the first one it then adds on the bottom. I’m not sure if it is time based or whether it skips a fixed number of activities, but I have missed runs because they were not shown.
In a couple of instances I have gone through all my friends seeing what runs they have logged to make sure I haven’t missed any.