I was under the impression that all preserved collection routes required you to use classic controls rather than the immersive. However, when I did a service on Main Spessart Bahn, I had been switched from the classic controls I used from the tutorial to immersive. Is this normal?
It is normal. Preserved collection tutorials are scripted to the classic control scheme as you are having to do specific actions. It says it will change back whenever you see that message. You are free to use classic or immersive for preserved collection services.