MPEG-4 Part 17 (a.k.a. TX3G ) subtitles

I'm not sure of this is a bug or an unsupported feature... I was going to use MKVToolNix to remux some MPEG-4 Part 17 (a.k.a. TX3G ) subtitles in a .mp4 file - but, MKVToolNix did not see the subtitle streams. HandBrake and VLC have no problem with this type of subtitles.