It's not a bug, and no, post-200m exp does not count for hiscores.
What happened with your rank was that some players, who had achieved 200m magic earlier than you and were unlisted from the hiscores due to inactivity, logged in again, thus making themselves listed on the hiscores again and pushing your rank back. The more players there are between you and rank 1 on the skill, the more inactive players there are, and thus the more inaccurate your permanent rank is when displayed on the hiscores. As such, only Jagex knows from their data what your true permanent rank is, unless they made it visible to you as well.