Well, the policy is that complimentary MCE seats "also applies to up to 8 passengers traveling in the same reservation with you." By changing your class of service, you can no longer be traveling in the same reservation as the rest of your party. You should have moved one of your companions into first class, kept your reservation in coach, and then just taken the FC seat yourself.

https://www.aa.com/i18n/aadvantage-p...istancedetails

To be honest, I don't like the "same reservation" rule because there are a number of reasons why passengers may be traveling on separate reservations, including IRROPs where the computer splits the reservation. But it's understandable that the benefit would only apply to passengers traveling together, and sitting in separate cabins is not traveling together.