Please be aware that there are two types of Moonstones in the game: the free ones and the purchased ones.
The purchased Moonstones bought with real money are platform-specific. This means you can only spend these platform-specific Moonstones on the related platform.
The free Moonstones you collect in-game are shareable across all platforms (except on PlayStation consoles) and when you spend them on one platform, the spent Moonstones are also deducted on the other platform.
When you run out of the purchased Moonstones, you automatically start spending the free Moonstones that are deducted on both platforms.