So the specs are this ( taken from FP3 Product page) and AAC is clearly included I would say
Supported audio codecs
- PCM playback, AAC/AAC+/eAAC+, MP3, WMA (v9, v10), WMALossless, WMAPro 10, AMR-NB, AMR-WB, FLAC, ALAC, Vorbis, AIFF, APE
I use some not very high end tech in ear Huawei buds and with those I dont have the greyed out options, if its headphone or FP fault or combination, I dont know. I have the FP3 (dont think that makes a differnence) and Iode OS (did not check with FPOS when I had it installed)
AAC is available and enabled in the Headphone settings on the FP, so you dont have this at all? Can you test other headphones?
Do you probably need to enable something on the headphones? Did you use them first with the Ipad and later on only with FP3? can you reset the headphones?
Maybe a logcat as indicated here can give some hints (you could use e.g. the App Matlog).