it requires a dash before the disc0x https://support.plex.tv/articles/naming-and-organizing-your-movie-media-files#toc-2
i.e.
The Lord of the Rings The Fellowship of the Ring (2001)-disc1.mkv
The Lord of the Rings The Fellowship of the Ring (2001)-disc2.mkv
Tip: join the two files, because Android clients still don’t work with multi-part videos [HowTo]: joining multi-part movies files with MKVtoolnix GUI
