Issue
By default, the simulator is setup to pause whenever you focus an application outside of the sim.
Ridiculous, I know!
Solution
- Launch your simulator
- Go in your simulator's Preferences
- P3D: Options > Application > Uncheck "Pause on task switch" (1st toggle, top left)
- FSX: Settings > General > Program > Uncheck "Pause on task switch" (2nd toggle, top left)