Yes, Plex is missing support for Composer, as well as some other tags, which does indeed hamper its ability to manage classical music. But you can manage it with some work.
Forgive me if the following is overly basic – I don’t know your level of expertise.
Plex basically has two ways to identify your albums - by the folder/filename structure, or by embedded tags. If you’re depending on the folder/filename structure (Plex’s default), then you might look into whether your files are organized in such a way that Plex can interpret them.
note: Even if the files are well organized, if there are embedded metadata tags in the folders and they aren’t correct or consistent, it may be confusing Plex. So you might look at the tags in these files and either correct them, or remove them.
If you have Plex configured to prefer embedded tags, the problem may be in your tags. This How-To article should help.
