Could Track N° be defined on 1 digit only ?

[simoncn]

The problem with question marks is not related to 1-digit track numbers. It is caused by a bug in older versions of JRiver that caused tag values to be written to FLAC files using an incorrect format (trailing NUL character). If you have a recent version of JRiver (18.0.153 or later), you can ask it to rewrite all tag values from its database into your audio files and this will solve the problem.

Changing a tag value (for example track number but could be anything) causes the tag values to be rewritten to the file and this is why it removes the question mark from this file.