If you choose your app to be Assigned to All Desktops, like:
You won't be able to select it from Mission Control.
Check this video: https://youtu.be/6WHoD_bTn9A
If you set Assign to: None, Mission Control is working great, but the app is locked to the current Space.
How can I fix this? It's really annoying, because I have lots of apps, that I want to be available in every Space.
Hope somebody knows how to fix this.