Why is the subjunctive wrong here?

Question

"Tu coche no es muy nuevo aunque funcione/funciona estupendamente. (Your car is not very new although it works beautifully.)"

I chose the subjunctive "funcione" here because both the speaker and the listener would know about the car. But this was marked wrong. Why would the indicative be correct?

PS I just read your answer below that the speaker is simply making a declaration. In that case, how do we distinguish this from the case of the speaker stating shared information? It seems that both answers could be correct depending on how one interprets the speaker. This makes it hard to know which answer the system considers to be correct.

Accepted Answer

Hola Marcos
The hint says "the speaker is declaring that the car works beautifully", therefore we need to use the indicative (funciona).
However, if the hint were to say that "the information is known by the listener" or "it is a future idea", then we would need to use the subjunctive (funcione).
Un saludo
Silvia