The automatic track log recording feature looks for GPS data up to 30 minutes prior to takeoff and adds that data to the recording. It will also record for about 15 minutes after landing. If your iPad has a GPS signal at your actual location 30 minutes prior to connecting to your sim and then taking off, then you will get GPS data from your actual location. Once you start your flight simulator, the recording will then receive and record your simulated GPS location.
To prevent this undesired recording behavior, start your simulator and connect it to ForeFlight. Once you have an active simulated GPS signal, restart ForeFlight like this:
STEP 1. Exit ForeFlight and go to the main iPhone or iPad home screen.
STEP 2. Press the "home" button at the bottom of the iPhone/iPad twice in a row (Double Click). Apps in use will be displayed in the center of the screen. If they don't appear, try again until they do.
STEP 3. Find the ForeFlight icon by scrolling through the apps.
STEP 4. Touch the ForeFlight app and swipe it upwards off of the screen. This will stop ForeFlight from running in the background.
STEP 5. Tap on the top of the home button to exit.
STEP 6. Tap the ForeFlight Mobile icon to restart ForeFlight.
After the above steps are done, you should only get simulated GPS data recorded to your Track Log from the time you restarted ForeFlight as long as the sim is the only source for GPS data.