1. Say create a dummy01.mkv MKV file with FFmpeg. Say 10 secs, no audio. I just based the code below from a Stack Overflow post years ago, as I am just a regular user.
3. Open that new dummy02.mkv with Mp3tag. Say put a value in the genre box, say comedy, then save. Then the Title is gone or deleted from the MKV file. Such "Title" is labeled as Movie name in MediaInfo, while in Windows' File Explorer, the column title in Details View is also Title.
I had a previous similar post. Back there, MKVToolNix GUI and its mkvpropedit.exe were used instead. Same issue of the Title getting auto-deleted by Mp3tag after modifying an input box and then saving.
Please, please check on this. Numerous of my MKV files lose their titles all at once after modifying with Mp3tag.
But the TITLE is in fact inside of the MKV file, as proven by FFprobe, MediaInfo, and Windows File Explorer's Title column in Details View. If opened by MKVToolNix GUI, it's also there.
The FFmpeg command of putting the Title into the Matroska file is just the same for other formats like Mp4 and Mp3 files.
All of them are compatible with each other. If remuxed by MKVToolNix, that Title is preserved.
I have been batch tagging Matroska files before with Mp3tag, and such issue did not exist back in time. If I try to look back hard, my guess is 2022-2023 range when the issue manifested. I have old MKV files since the 2019, and no Title auto-deletion issue back then.
I just tested with MPC-HC, MPC-BE, VLC, KMPlayer, SMPlayer, and mpv (mpv.exe), and all of such players displayed that Title in their respective titlebars.
Nonetheless, Mp3tag shouldn't be auto-deleting whatever the value is in that Title. Especially if a different input box is modified by Mp3tag, just like in my example earlier, the genre box.
And you are right:
Mp3tag seems to write the content back in way that - for example ExifTools - can't read it anymore.
The TITLE seems to get lost completely.
As I don't use Mp3tag for MKV yet, I created a test batch file including all the Tags from the ffmpeg Public Metadata API to create a dummy05.mkv based on @Wakaku's dummy01.mkv:
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.
With the help of this Open Source tageditor I was able to fill the "default" mkv tags in the Levels 30, 50 (and 60 collection level) and this is how they look in Mp3tag:
So you are right (as expected):
The L30 Artist would be filled into Mp3tag ARTIST
The L30 Title would be filled into Mp3tag TITLE
The L50 Artist will be filled into Mp3tag ALBUMARTIST