I can think of a couple of possibilities. One is that, even though you have the ID3 tags set, Plex may not be completely configured to prefer them. Follow this guide to make sure.
The second is that there may be an error in your tagging that you haven’t noticed. For one, do the single-digit track numbers have leading zeroes?
Can you post a screenshot of your tag editor showing the filename, track number tag, and Title tag? Here’s a shot from one of mine:
