It seems MB (web edit) does not support thin space (U+2009) and narrow no-break space (U+202F).
You can enter (or paste) it for track titles in the Track List tab, still have it in the Recordings tab but it’s substituted in the Edit Note tab by a regular space.
The narrow no-break space is recommended for some cases, e.g. grouping of digits (German) and would be fine to have.
Is there a special reason for not supporting it?
Or is there another reason for replacing it by a regular space?