Surround AC3 stream inside MP4 recognised as 3F2M

Note: this might be a ffmpeg issue, but VLC is the only player I managed to have surround over HDMI with.

I have to convert AAC streams to AC3 because my home cinema can't decode it.
With a mkv container, the converted AC3 stream plays correctly in surround, by going to Tools -> Codec Information (Ctrl+j) it properly shows 3F2R/LFE channels.
But with a mp4 container, it plays in stereo and Codec Information shows "3F2M/LFE".

Steps to reproduce

1. download an AAC surround test: wget https://archive.org/download/5.1SurroundSoundTestFilesVariousFormatsAACAC3MP4DTSWAV/5.1%20Surround%20Sound%20AAC%20Test.mp4
2. convert to AC3 inside a mkv with ffmpeg: ffmpeg -y -i 5.1\ Surround\ Sound\ AAC\ Test.mp4 -map 0 -c:v copy -c:a ac3 ac3_ok.mkv
3. convert to AC3 inside a mp4 with ffmpeg: ffmpeg -y -i 5.1\ Surround\ Sound\ AAC\ Test.mp4 -map 0 -c:v copy -c:a ac3 ac3_fail.mp4
4. play ac3_ok.mkv and ac3_fail.mp4. Note: you MUST play the files on a surround device, otherwise both files show "3F2M/LFE" - another bug?