This is, unfortunately, one of the most glaring deficiencies in Plex’s music support. They do not account for the composer, or several other tags that are essential for good support of classical music.
There is a feature request that addresses much of this. I encourage you to vote for it, but at this point I wouldn’t hold my breath.
Someone else recently asked the same question. Here is my response, which includes how I have worked around the problem:
