WordPress reduces the full size image and uses it as the original

This behaviour was newly introduced by WordPress with version 5.3 (released on 12th of November 2019). So if you updated to that version, WP is behaving according to design. The announcement post for the new version only mentions it as

including improved large image support for uploading non-optimized,
high-resolution pictures

in the section “Block Editor Improvements”. More technical details can be found in trac filed under Ticket #47873. This change also brought a filter big_image_size_threshold which can be used to control this feature.

function wpse352820_big_image_size_threshold( $threshold, $imagesize, $file, $attachment_id ) {
    return 4096;
}
add_filter( 'big_image_size_threshold', 'wpse352820_big_image_size_threshold', 10, 4 )

Just return the value in pixels you want the pictures to have as maximum on the longer edge. If you return false in the filter, the feature should get disabled entirely, in short this can be written as follows:

add_filter( 'big_image_size_threshold', '__return_false' );

Each of these code snippets can be used by simply dropping it into your theme’s functions.php or make a plugin out of it.