AAC 5.1 audio becomes 4.0

Hi.

I have my HTPC connected through HDMI to a Yamaha RX475 with a set of 3.0 speakers. I have Kodi speaker configuration set to 3.0, and passthrough enabled, as well.

Movies with AAC 5.1 audio tracks are sent as PCM 4.0 to the Yamaha amplifier. Therefore, the central speaker remains silent.

However, DTS and AC3 tracks are played correctly, and the central speaker works.

I have tried with Jarvis 16.1 and Krypton RC3, with identical result..

What I am doing wrong? How could I hear the central speaker when an AAC audio track is being played?

Thanks.

I am attaching kodi.log

Jarvis: http://pastebin.com/8UHqcY61
Krypton: http://pastebin.com/gzsFzkLK