It looks like the image that lightboxes (Hoppip) has a height of 501px, that’s just 1px over the default max image height value of 500. So that lightbox lets you view it at the full 1px taller size.
The other image (Growlithe) is only 498px tall, so it doesn’t lightbox because it can be posted at the full size.
So that’s working as expected.
One thing that makes it a little bit of an odd experience is when images are manually scaled down in the post (in this case both images have been scaled to 50%). It seems like if we have the original image, we should lightbox it so people can easily view it at full scale…