Ah that makes sense now, I remember seeing something about that update but hadn’t realised how it might affect my site. I’ve entered a new image size in my functions file but unfortunately I’m having trouble regenerating the thumbnail for that problem post. Using the Regenerate Thumbnails plugin mentioned in the your link I go to my media library and locate the images I’m using in the featured image, click Regenerate Thumbnail but I get the error the:
ERROR: The fullsize image file cannot be found in your uploads directory at 2020/05/filename.jpg. Without it, new thumbnail images can't be generated.
The original image is definitely there as it’s showing in the post as the featured image. I’ll try re-adding the image to the library and see if that works.