By default, Safari will block all third party cookies, effectively preventing you from reporting as an authenticated Marker.io user.
To fix this, in Safari, simply go Preferences and then, under Privacy, make sure to uncheck Prevent cross-site tracking and Block all cookies.
If you're using Safari Mobile on your iPhone, go to Settings > Safari. Under Privacy & Security, Disable Prevent cross-site tracking and Block all cookies.
That's it! You should now be able to report bugs as a logged in user.