Steps to reproduce:
1. Edit an existing, but incomplete release (or add a new release with just a part of the total tracks, it's not common but it can happen).
2. Open the track parser and paste the missing tracks (making sure that the track numbers increment on the existing ones).
3. Result: the actual track number values are ignored and the parsed tracks substitute the existing ones. You can't append new tracks to an existing medium.
This can become a major pain when adding VA releases with a long tracklist and many new artists.