The new system doesn't count for ranks, so it shouldn't have anything to do with the fact you got a top50 on a new map.
What I've heard is that ppv2 is slightly farmable at lower ranks, so if you're getting A-ranks or S-ranks on more difficult stuff, you'll be carried up to the rank of others with the same capability. The ppv2 system is also fairly forgivable; you can't lose pp by playing badly (unless you get a new high score on a map with lower accuracy, but that's a limitation of the current system rather than an intended feature).
What I've heard is that ppv2 is slightly farmable at lower ranks, so if you're getting A-ranks or S-ranks on more difficult stuff, you'll be carried up to the rank of others with the same capability. The ppv2 system is also fairly forgivable; you can't lose pp by playing badly (unless you get a new high score on a map with lower accuracy, but that's a limitation of the current system rather than an intended feature).