Enabling subtitles forces AC3 to AAC conversion, despite AAC being available



Here's my situation: I have an m4v movie file with dual audio streams. Stereo AAC and 5.1 AC3. Also there is an external UTF-8 srt subtitles file.
If I play the movie on my iOS devices without subtitles, I correctly receive a smooth Direct Play with H264 and AAC. If I enable subtitles, even though I have the AAC audio stream selected, the server chooses to take the AC3 audio stream and transcodes it to AAC, despite my selection.

This makes no sense to me. Could anyone explain a reason or a solution to this problem?
I would like to have my files with dual audio streams (stereo AAC & 5.1 AC3) to ensure maximum compatibility if I ever get 5.1 setup.
Thank you.