This is because YouTube has once again changed its design and now when you add videos to your favorites it gets added to the end rather than the beginning. Why? Well favorites are no longer favorites. The old favorites is now gone and upon creating a new YouTube account you will notice that there is no option to save a video to your favorites when you click on Add to, rather it just displays ‘Watch Later’. The old Favorites is now a list and when you add a video to a list it gets added to the end.
This means that when saving a video to your favorites you need to check the box that says ‘Add videos to top of list’ and your video should appear at the beginning of your favorites.