MP4 tx3g subtitle stream not carried to MKV

vlc correctly plays a certain MP4 file with soft tx3g subtitles, identifies them as stream 2. FFMPEG has no trouble to extract them as an SRT file.

But when I drop the file in MKVToolNix GUI v20.1.0, that stream isn't recognized, and of course isn't carried over into the converted MKV file.

Is this a known bug? Is it a structural problem with the MP4 file? Is there a workaround?

All the best --

djcg