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…
I’m working with images that are around 500px tall and I guess some are a bit over and some are a bit under. I set them all to 35% scale so they fit together side-by-side but also with the idea that if you want a closer look you can use the lightbox like a gallery.
In this example only the first image lightboxes and also creates this strange positioning: