If a mp3 file has a Tag 2 Mood empty, a new added Mood tag diappears after Save.
If the Tag 2 Mood already exist it may be edited and works fine.
Try to add a Tag 2 Mood to the attached file.
If you look at id3v2.4.0-changes (id3.org) (5. New frames) or the frame table in the Kid3 handbook, you can see that TMOO (Mood) is a new frame in ID3v2.4.0, i.e. it does not exist in ID3v2.3.0. If you want to use Mood frames, you have to use ID3v2.4.0 frames, you can change this in the settings of Kid3. TagLib, the library which is used for ID3v2.4.0 tags, will discard unsupported frames when an ID3v2.3.0 tag is saved (in function ID3v2::Tag::downgradeFrames()), this is what is happening here.
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If you look at id3v2.4.0-changes (id3.org) (5. New frames) or the frame table in the Kid3 handbook, you can see that TMOO (Mood) is a new frame in ID3v2.4.0, i.e. it does not exist in ID3v2.3.0. If you want to use Mood frames, you have to use ID3v2.4.0 frames, you can change this in the settings of Kid3. TagLib, the library which is used for ID3v2.4.0 tags, will discard unsupported frames when an ID3v2.3.0 tag is saved (in function ID3v2::Tag::downgradeFrames()), this is what is happening here.
Thank you very much, you've solved my problem!