Maybe I understand how it is…
AAC is not possible to passthrough, so Enable passthrough option doesnt have affect on this particular codec. So even when I have “passthrough enabled”, it doesnt means all audio goes passthrough (not as I though: global passthrough), but only supported audio goes passthrough. Guessing that supported codecs are those, who have option in Passthrough settings… (?)
Because that AAC just got decode all times, and when I setuped 2.0/5.1 pass/not pass? Just guessing, as I think when I enabled 5.1 output and use 7.1 AAC, AVR shows PCM, and shows it gets 8 channels (not practical problem, as AVR has setuped correct number of speakers connected, but worth mention/question this)