Version: 2.37a
Apologies if I'm wrong about this, but...
... it appears that using %_filename% to cross-reference text file entries doesn't work yet %_path% does when the number of lines in the text file doesn't match the number of files selected.
Using %_filename% (and just the file name without an extension in the text file) in this scenario results in Mp3tag ignoring the file name and applying the tags to the first n files that are selected, where n is the number of lines in the text file.
Using %_path% (with the full path in the text file) works as expected.