wp_title filter takes no effect on the <title> tag

Not sure if its necessary to inject the variable, but try this.

function change_the_title($title) {
    return 'My modified title';
}
add_filter('wp_title', 'change_the_title');

I finally found out that the WordPress core code was changed, see the below piece of code.

/**
 * Displays title tag with content.
 *
 * @ignore
 * @since 4.1.0
 * @since 4.4.0 Improved title output replaced `wp_title()`.
 * @access private
 */
function _wp_render_title_tag() {
    if ( ! current_theme_supports( 'title-tag' ) ) {
        return;
    }

    echo '<title>' . wp_get_document_title() . '</title>' . "\n";
}

So, after 4.4, the core do not inject the wp_title result into the header <title> tag, but do the same thing with a new function wp_get_document_title.

So instead, we can do the same thing by:

1. change the title directly:

add_filter('pre_get_document_title', 'change_the_title');
function change_the_title() {
    return 'The expected title';
}

2. filtering the title parts:

add_filter('document_title_parts', 'filter_title_part');
function filter_title_part($title) {
    return array('a', 'b', 'c');
}

For more, see the details here: https://developer.wordpress.org/reference/functions/wp_get_document_title/

PS: Looking into the source of function wp_get_document_title is a good idea, the hooks inside which tells a lot.