The CAD renders leaked a few months ago might be showing the regular Mate 40, while the Pro version could feature a new design, or Huawei just decided to change some design elements in the meantime. In any case, the sneak peek render posted by He Gang himself shows that the rear camera bump is not circular anymore, and the volume button appears to be back.
https://www.notebookcheck.net/Huawei-Mate-40-official-sneak-peek-render-reveals-unique-rear-camera-bump.497997.0.html