This is not a nerf it's always worked that way. It's working better than advertised because it only promises to take you to 16 (by giving you the xp needed to get from where you are to 16). Everything beyond that was not working as intended but the team was kind enough to let it keep working that way. So you needed less xp to get to 16 your uintended bonus xp was also smaller.
Yes, the higher level you are when you use the stone the less benefit you get out of it.