add_image_size adds size, but doesn’t upload if source resolution is same as resize resolution

I have added an image size

<?php add_image_size( 'test', 300, 300, true ); ?>

Uploaded an image of exactly 300 x 300px,

Then got the image

<?php wp_get_attachment_image_src( $id, array(300,300) );?>
or
<?php wp_get_attachment_image_src( $id, 'test' );?>

Works both ways. Maybe you did not define the image size correctly in functions.php