ffmpeg + libfdk_aac + vbr => wrong "BitRate_Mode"
The issue was caused by ffmpeg/libavformat/movenc.c::mov_write_esds_tag, it can't find max_bitrate. To solve it you should specify audio bitrate -b:a 128k along with -vbr 4. Then mp4's ESDescriptor would have info about max_bitrate and mediainfo and decoders would know that audio stream has variable bitrate mode.