If I open multiple playlists in one operation (e.g. Explorer RC mp3tag), the bar goes to 100% during the first playlist and stays there for the remainder.
The same problem exists with multiple folder open e.g. select two folders in Explorer and drag into mp3tag.
Unfortunately, Mp3tag can't predict the number of actual files.
I'm surprised it can't just count them first, as it does e.g. upon one folder containing two sub-folders.