It seems that ffmpeg is writing TITLE to the Segment Title and not to the Tags. Mp3tag only reads Tags and Chapter information from Matroska. I'll look into this.
@LyricsLover Matroska has a very advanced tagging system which seems to be not really supported by ffmpeg (at least not via these command line parameters).
If you're interested you can read Matroska → Technical Info → Tags where the concept of Target Types is discussed. It allows for the same tag field, e.g., artist to be stored for different levels. In this context T=50 means the album level and T=30 the track level. Tag Field Mappings – Mp3tag Documentation shows how Mp3tag maps, e.g., an ARTIST field on T=50 to the ALBUMARTIST field.