There is nothing much to say about. WordPress silently added an image rescaling functionality without any configurable UI. It automatically scales uploaded images when it thinks they are too “big” and thus “unoptimized for SEO”. The Football Manager game logs therefore have blurry images, while the Europa Universalis ones didn’t, because they were uploaded before the update that brought this functionality.
Here is the complaint: shouldn’t the default behavior be as straightforward as possible, and leave tasks like SEO to dedicated SEO plugins? That way, people who doesn’t care about SEO (like me) won’t need to worry about anything.
To make things worse, WordPress still keeps the original big images. There is just no button to use them. They just sit in a purposeless back store, wasting the space for no purposes. Speaking of which, vanilla WordPress should spend more time on media management, like, allowing me to organize my images with folders. Just a few years of Football Manager is already a chore, and I can’t understand how can people use WordPress as actual photo galleries. That is an area I am not comfortable with plugins, since I feel it is tightly coupled with all underlying mechanisms and is best not touched by any outsider.
OK, back to topic. I added this to my subtheme.
add_filter( 'big_image_size_threshold', '__return_false' );
Problem solved, but it’s a problem that should never appear in the first place.