Shinigamis could always leap very far, obviously breaking the physics of gravity from the start. But I also feel like combat was based at the ground level much more earlier in the series. This and the fact that Rukia and other Shinigami were usually shown perched on roofs and telephone poles gave the impression that they could not just levitate.

I really don't think it's strange in itself, but sometimes the characters act as if there is ground under them in the air (when standing up or getting knocked around /falling in particular), and that's awkward. I'd chalk it up to the animators being used to just down ground-based stuff.