WordPress Core will always crop the image from both sides and the middle, so either it will be cut on the top and bottom or on the left and right hand side.
The theme defines the image size which is used for the featured image in posts, but does not control the image cropping, that is part of WordPress itself.
You have basically two possibilities:
1) Use a plugin to change the defined featured image size to a size which better fits your needs. You can do that with the Simple Image Sizes plugin: https://wordpress.org/plugins/simple-image-sizes/
You can also specify a height of 99999px to ensure that the image is never cropped on the top or bottom size.