On first firing up MP3Tag, 'Select all' works OK.
The circumstance under which it doesn't work is when the focus is no longer on that window.
Suppose a selection has been made (you just wanted to amend one tag), then you click in the 'Tag Panel' on the left. The highlighted item remains highlighted but the focus is no longer on that window.
Now 'Select all files' does nothing.
The other options (invert and unselect) work OK however, so you could 'Unselect all' followed by 'Invert selection' if you like doing things the long way.