Aspect ratio issue with x.265 media?

I've had issues with some x.265 media and Plex for some time. Some movies are played in the wrong aspect ratio. Not entirely sure if it's Plex problem or Handbrake issues but VLC has no problems with the x.265 media I've tried so.
I've attached a short clip from a kids-movie where the problem appeared. It's encoded in both x.264 (problem-free) and x.265 (which is looking bad in Plex).
Hints?