Yes, now that FNS is over it seems even more strange that she didn't attend. She could have sung Sunset for example, most of the other acts performed older songs. My guess is that she didn't have time. FNS is more than 3 hours, right? And all the acts sit there and listen to the other acts. So maybe she doesn't have time right now to do nothing for 4 hours?
On the other hand that explanation is not really that convincing. Very mysterious. *sigh*