A column with %_folderpath% in 'Field:' refuses to be edited i.e. is read-only in the tracklist.
Help says:
Field
This field is optional. The placeholder of this field is used for editing the content of the field through the file view - if this field is empty, the column is write protected.
This field is optional. The placeholder of this field is used for editing the content of the field through the file view - if this field is empty, the column is write protected.
You can use placeholders for tag fields, %_filename_ext%, or %_filename% here.
One fix to the problem would be:
...
You can use only placeholders for tag fields, %_filename_ext%, or %_filename% here.