Turns out it's worse than that - same problem occurs with the Plex client too. It applies to any video source that isn't 16:9.
Cinema-style video (say 14:9) looks like it's stretched vertically to fill the screen, and then the black letterbox bars applied over the top. This also means that not only is the image distorted, but if there's subtitles they have the black letterbox bars over most of the text!
There's a lot more about this issue with reports from on around the world here:
Hisense support are worse than useless on this issue. But it reflects badly on the Opera TV platform.
Can anyone from Opera please provide some comment on this?