Which MP3tag version are you using?
Please note that V3.25
has some fixes in respect to UTF8:
[2024-02-03] FIX: writing UTF-8 encoded export files produced wrong 4-byte character encoding for some Unicode characters. (#63556)
[2024-02-02] FIX: writing UTF-8 encoded *.m3u8 playlists produced wrong 4-byte character encoding for some Unicode characters. (#63556)
[2024-02-02] FIX: error when reading UTF-8 encoded *.m3u8 playlists with Unicode characters encoded using 4-bytes.
I don't know, though, if these also apply to cue files.