1 pointSeems 'html, body' is correct but for desktop only, 'window' is used for mobile devices. See here: http://blog.jonathanargentiero.com/jquery-scrolltop-not-working-on-mobile-devices-iphone-ipad-android-phones/
1 pointI'm not sure what else it would be, there might be another animation playing still. It seems like there are competing animations. The fact that it doesn't do it if you wait (presumably until after any animations finish) suggests that's the issue.