Can't change Handbrake audio bit rate
What tool do you use for showing the Audio bitrate?
Try MediaInfo to make sure that Handbrake is the problem here.
But as far as I can see, you aren't doing anything wrong (besides setting the audio bitrate this low ;) ).
The avcodec AAC encoder also has quality settings, you can try these out as well (Couldn't hear any difference for settings 3 and higher, but was able to hear some slight artifacts at 2, never tried 1).
For best compression use MKV container with opus audio and x265 video encoder.