In most cases, Plex only shows the track artist if it is not the same as the album artist - for example “album artist featuring another artist”
There’s also a long existing bug that only affects mp4/m4a files where those type of files always show the track artist, even if it is the same as the album artist. The bug is probably only noticeable if your music library is configured to use local metadata.
Personally, I would prefer the track artist always be visible, but I doubt there’s much interest.