This is intentional to prevent tall images from dominating the discussion (the full image is still available on click).
We consider it a safe default for most sites, but if your community wants to post tall images you can adjust it.
The setting that controls it is called min ratio to crop and the default is 0.45; the image you posted here is 1531x3962, so when width / height that’s 0.39 and it gets cropped. If you want similarly sized images to avoid being cropped, you could decrease the ratio a bit (maybe 0.35?) and see how it goes.