OK, I tested it again with the PC version of VLC. Obviously it's the crop setting that is 16: 9 and not default. Unfortunately, you can not adjust this setting, so the VLC player on the FireTV stick is unusable.
I have the same problem. For 21: 9 videos the black bars are cut off at the top and bottom and the 21 area is compressed to 16. So the films are not viewable.