I use following command in server 2.1 beta2 for imx50.
ADD 1 FILE "/test.mxf" -b:v 50000000 -codec:a pcm_s16le -codec:v mpeg2video -pix_fmt yuv422p -alternate_scan 1 -g 1 -minrate 50000k -maxrate 50000k -timecode 17:28:07:10 -metadata creation_time=now -color_primaries bt470bg -color_trc 1 -colorspace bt470bg -vf setdar=4:3 -f mxf_d10 -d10_channelcount 8
I think you need -vf setdar=4:3