This is more of a UI fix, than a feature request.
I use a 4K monitor, and often have Strava opened on a website in one of the 4 corners of my large display.
When I click on the full screen option on any Map within any activity on Strava, the map goes full screen on my entire 4K display, covering all other windows as well. The expected behaviour is that the Strava Map goes full screen on only that browser window.
It is up to the user to maximise the browser to the full 4K size, and then click full screen on Strava, if they want that big size of a map. Strava may wish to add both options, but the default should be for the given window, not the entire display.