Cannot get full thumbnail size using the_post_thumbnail

I cannot get the full (unmodified) image size using

the_post_thumbnail( 'full' );

I have also tried:

the_post_thumbnail();

It still returns the cropped image. I can see that the unmodified size is uploaded on the server and there is no problem with that.
I checked using

print_r(get_intermediate_image_sizes());

And it returns following sizes:

Array
(
    [0] => thumbnail
    [1] => medium
    [2] => medium_large
    [3] => large
    [4] => custom_size1
)

Why there is no full size? Has something changed in the WordPress recent versions or is it something else? Thanks.

Edit: I have figured out that it is the WordPress srcset which is adding multiple image sizes in the URL and it uses the smaller image.

Try to add custom thumbnail size with unlimited(very big) hegiht and width in functions.php with following code:

add_theme_support( 'post-thumbnails' );
add_image_size( 'custom', 9999, 9999 );

It will not crop any images. So you can use it as the_post_thumbnail( 'custom' );

This method looks rough but you can use if other ways don’t work. And if you already have images, you can use regenerate thumbnails plugin.