It exports exactely what is in the preview, so not 16x9.
When played in MS media player,or any other video viewer, black bars show to compensate the deviaton from 16:9
But even if it would be 'solved' during export, the result would be different from what is shown in the edit/preview.
Using 3.14 for the same source file works perfect, so it really looks at if it is a bug.