I completed a mission for a smart device app, but I don't see the Platinum Points in the corresponding app.
You may be accessing the smart device app using a different operating system (OS) from the one you used to earn the points.
Smart device app Platinum Points are tied to the OS of the device you used to access the corresponding app. If you use both iOS and Android devices to access a smart device app, the totals will be different depending on which OS you're using.
However, the My Nintendo website will always display your total number of smart device app Platinum Points across all operating systems. For the most accurate information about your point total, please check My Nintendo.