I've located what caused, at least, coming-and-going of playlist selection box.
While having lots of trouble with this beta version, one of the most annoying thing was that playlist selection box was constantly coming and going, sometimes with no apparent reason. Then it occurred to me that whenever I updated the new beta version I always left the official version of mb_TagTools.dll in the same folder under the filename of mb_TagTool.dll for easy restoration of the plugin's functionality after crush (by swapping the filename of the old and new dll file). A little while ago I deleted the file, and then immediately noticed that the box was back and has stayed stable. Also the auto-applied preset has been working fine so far. I'll report back if that will really do the trick. BTW how come a dll file with different name can affect the plugin's functionality?