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
mainfolder->
folder1->
song1.m4a
song2.m4a
song3.m4a
folder2->
song21.m4a
song22.m4a
song23.m4a
textfile
song1.m4a,artist1
song2.m4a,artist2
song3.m4a,artist3
song21.m4a,artist21
song22.m4a,artist22
song23.m4a,artist23
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.