Actually this is not cropped image by theme. This featured image is showing in it’s full size i.e. 1200 x 456. This is the reasons, you are thinking it is showing properly.
Can you help me with that?
WordPress crops image automatically based on the theme image size settings.
You are uploading a Landscape image with the width of 1200px. I will suggest to upload a square image with the dimension like “600×540” so it will crop the image to fit your needs.
Please see the theme screenshot and the modified image dimensions of your featured image.