A very minor improvement could be to clear the filter when changing directory or when re-opening MP3Tag.
And if it is on purpose that the filter is not cleared (maybe this was a feature requested previously?), this could be announced with a modal message box, e.g. "Filter not clear; not all songs/files are shown" or something like that.
It happened to me today that I reopened several times a directory, restarted MP3Tag, because no files/songs were shown, while I was sure that there were thousands of files in the directory.
And finally I noticed that there still was a word in the filter box (which is in my case placed at the bottom of the window, and with my small screen resolution, not always easy to notice), and because no files matched, none were shown ...
Ok, maybe I am stupid, or not attentive enough, but if it happened to me, it could happen to other users. 
And for me, a filter is linked to the content of the navigation window, so if that changes, it does not seem illogical to clear the filter. In any case, the history is saved, so if you want to re-apply a more complex filter you created, it is easy to reply it.
Another suggestion (either cumulative or in place of the above), is to give a visual clue in the navigation window that a filter is applied. I am not a HMI or usability expert, but the column headings could have a different color, e.g. green if the filter did find a match in a column, and red if not. Or if that is too complex, or too much resembling a Christmas tree, the entire row with column headings could become light green if matches are found, and light red if none are found.
So it is visually clear in both cases that a filter is active.
Or probably there are some standards in other tools/software on how to show a filter is active ...
