The same happened to me when I was playing as Ryazan. It costed me a lot of monarch power to keep Muscovy loyal by developing their provinces. However, I managed to integrate them in the end which felt so damn satisfying.

When I was playing as Muscovy a couple years ago, I was forced into a PU with Ryazan that was uncontested. They were allied to Poland-Lithuania, so it was pretty hard winning my independence.