When encoding MP4 files from Media Encoder or Premiere Pro the exported file is missing the SAR and DAR values.

This can be easily check with ffprobe and this command:
ffprobe -v error -showformat -showstreams file.mp4
[STREAM]
index=0
codecname=h264
codeclongname=H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10
profile=Main
codectype=video
codectimebase=1/50
codectagstring=avc1
codectag=0x31637661
width=1920
height=1080
codedwidth=1920
codedheight=1088
closedcaptions=0
hasbframes=2
sampleaspectratio=N/A
displayaspectratio=N/A

As you can see above the encoded file shows N/A values instead of the expected SAR value 1:1 and DAR value 16:9.

I have of course tested with other source files and mp4 files from other vendors and they provide these values in the mp4 files they create.

Is this perhaps a bug or missing feature?

Hope for a resolution in the near future :-)