According to 9.2.2 of the Beförderungsbedingungen Personenverkehr: In the event that you have a 1st class ticket you are indeed entitled compensation, if the booked transportation should have had a 1st class but does NOT have it during you trip.
Thus, as the replacement bus is normally only considered 2nd class, you should be entitled to compensation. The height of the compensation would depend on your type of ticket: If you have a FLEX (full price) ticket you should get a refund of the difference between the 1st class and 2nd class ticket; if you have a SPARPREIS (discount price) ticket you are entitled to a flat 20€ refund. Either way you need some kind of proof that you were affected (e.g. some kind of signed letter from the staff etc. documenting that you needed to use the bus)
The easiest way would probably be to go directly to a trainstation and speak with the staff at the info-desk there; otherwise you could try contacting them via chat or the service hotline.