although %_filename_ext% is the record location in textfile->tag. If there are songs under subfolders, then textfile->tag expects absolute path with filename and extension record location reference by %_filename_ext%.
steps to reproduce
if all the songs under mainfolder is pulled inside mp3tag. then textfile->tag fails to import tags with %_filename_ext%,%artist%
but if subfolders are pulled individually say folder1, then the same textfile applies the tag with no problem.
Please review and fix it, if you consider this as a bug.