This doesn't seem like a problem with the library integration tool but there may be a workaround that can be done by your add-on. I use the library integration tool to pull some movies from another Kodi add-on. Your add-on worked and did populate the manage movies folder, but any strm file that was larger in size than 4 kilobytes could not be read and properly imported by Kodi. I'm somewhat new to this but I read about nfo files. Could your strm files include only links and a separate nfo file for metadata?
This doesn't seem like a problem with the library integration tool but there may be a workaround that can be done by your add-on. I use the library integration tool to pull some movies from another Kodi add-on. Your add-on worked and did populate the manage movies folder, but any strm file that was larger in size than 4 kilobytes could not be read and properly imported by Kodi. I'm somewhat new to this but I read about nfo files. Could your strm files include only links and a separate nfo file for metadata?