Hello, I have having a little trouble understanding the rules for the Flexpreis fare.
Let's say I:
1) Buy a Flexpreis ticket from Stuttgart Flughafen to Mannheim Hbf (via Stuttgart Hbf)
2) At Stuttgart Hbf, before boarding a train to Mannheim, take a detour to the Mercedes-Benz museum using the S-Bahn
3) Return to Stuttgart Hbf and transfer to a train to Mannheim

Is the round-trip from Stuttgart Hbf to the museum covered by the original ticket or do I need to purchase it separately?

Thank you.