How to disable this behaviour?
Add this to your functions.php:
add_filter( 'big_image_size_threshold', '__return_false' );
How does it work?
When a new image is uploaded, WordPress will detect if it is a “big” image by checking if its height or width exceeds the big_image threshold. The default threshold value is 2560px, filterable with the new big_image_size_threshold filter.
If the image’s height or width is above this threshold, it will be scaled down, with the threshold being used for maximum height and max. width value. The scaled down image will be used as the largest available size.
In this case, the original image file is stored in the uploads directory and its name is stored in another array key in the image meta array: original_image. To be able to always get the path to an originally uploaded image a new function wp_get_original_image_path() was introduced.
Source: Introducing handling of big images in WordPress 5.3 by Justin Ahinon