Blurry images when using add_image_size with crop

I've got the following image size: add_image_size( 'medium-thumb', 350, 350, true );

The images generated using the code snippet above are heavily compressed and turn blurry. This is the original image:

This is the 350x350 image generated by WordPress. Check the flowers and the side of the planter in the top left corner:

I have also added the following snippet to my functions.php but that doesn't seem to have made any difference add_filter('jpeg_quality', function($arg){return 100;});

Anyone has got a suggestion? Possibly an apache module causing this or a WordPress setting I'm overseeing?