VobSub .sub mis-identified as MPEG PS

With the provided sample, when giving the .sub to mkvmerge (v74), it appears to use the MPEG PS demuxer and the resulting file has no tracks (when passing the .idx file instead, the VobSub demuxer is used and the output file successfully contains both tracks).

Is this expected? I don't recall having this issue in the past but I may be misremembering. I haven't tried other samples yet. Older builds of mkvmerge (e.g. v53) appear to exhibit the same issue.