WP generates ugly inaccurate thumbnails

Hi,
i updated WP to 4.7, don’t remember why turned off all plugins to update them, and after turning everything on all my thumbnails have changed into something ugly. Now i can’t force WP to generate accurate thumbnails.

Here’s an example:
original

300×300

Thumbnails look like someone applied some filters to them, and cut out dark tones.

I tried regenerating thumbs with all plugins turned OFF, clearing my cache, creating custom JPG quality methods in functions.php, but nothing works.

How can i fix it? Wheres the code responsible for creating thumbs?