I have set the featured image on my blog post as before. Its dimensions are 1200×600.
When I look at the posts page on my blog, part of the featured image is cut off (the first post on the list). Other posts look normal, and I have used the same dimension on their featured images.
The new image sizing feature is cropping it to be 600px tall. It seems like you should just be able to remove that 600 value and it will look achieve the desired look.
It likely means our old image resizer wasn’t working on this specific image. If the image size doesn’t exist (old resizer failed), it will use CSS to resize it, which is what you were seeing.
It’s way better to use add_image_size to create your image size and set the Media Attachment Size.