Converting audio with cover art fails with MXF output.

[steipal]

It seem that converting audio (mp3 in this case) with cover art does not behave consistant. Here's some examples with the attached file:

ffmpeg -r 25 -i cover.mp3 -filter_complex loop -ar 48k cover.mov
ffmpeg -r 25 -i cover.mp3 -filter_complex loop -ar 48k cover.mp4
ffmpeg -r 25 -i cover.mp3 -ar 48k cover.avi
ffmpeg -r 25 -i cover.mp3 -ar 48k cover.mkv
ffmpeg -r 25 -i cover.mp3 -filter_complex loop -ar 48k cover.mxf

With the first two (mpv/mp4) I had to add -filter_complex loop to make it work. A simple -vf loop did not work. Not sure why cause I would expect them to behave the same.
The other two (avi/mkv) worked without any looping needed.
The mxf one fails with all combinations I've tried so far. It does create the file but it's one frame (0.04 duration) instead of the expected 14 frames, about 0.58 in duration.
"Cleaning" through piping is not an option. I need to do direct converting.

I'm using master built 12. Oct. 2022