Noticed it too, I have no idea yet. I use a wrapper around the native Fullscreen API's (https://github.com/sindresorhus/screenfull.js/) and it doesn't do anything fancy. Interesting, I'll have a look at it but I'll put it lower on my prio list if you don't mind.
Try scrolling down to the player, entering fullscreen, hitting Escape, you're back to the top of the page.