Just tried them both.
Croupier is a Starz encode. Starz screws up about 10% of their encodes aspect ratios and tend to not be of the best picture quality. I avoid Starz except for old TV shows and impossible to get otherwise movies.
Swimming Pool is a different kettle of fish as it is even in HD and should be the proper ratio (1:85 according to imdb). You can report bad encodes through netflix and you should do so. They do re encode some of them eventually.
To sum up, nothing on your end. Not Roku either. All Netflix.
It's not a bad encode. It's a 4:3 version of the movie. Reporting it won't help and would presumably take their time away from fixing actual bad encode. Instead always respond to their email surveys and rate the picture quality as acceptable.
HD doesn't in any way guarantee an aspect ratio by the way.
I'm not sure how much this applies to recent encodes but Netflix once said that most of their source materials were made for broadcast. The broadcast standard was 4:3 until very recently, and is still used on many SD channels.