I'm experiencing this as well, I play a song and get ranked in the top #500 and immediately drop 50 ranks. Even when I replayed it to get a higher accuracy and an even higher ranking on that map, I only got 1 ranking place back. Not sure if this is due to the actual map I played itself, but sure as hell is annoying I wasn't even attempting to get a top 500 score and I did, and it ended up reducing my rank...
Also, I actually gained PP and lost rank at the same time, could be some sort of delayed updating (like I was already losing rank from getting beat in PP or previous scores and the system just updates all that when I next get a high ranking)