add_filter('wp_title') doesn't replace my title tag (WordPress plugin)

If anyone else is having problems with this, it may be due to the Yoast plugin. Use:

add_filter( 'pre_get_document_title', function( $title ){
    // Make any changes here
    return $title;
}, 999, 1 );

I can't tell from the code you have provided, but are you using:

<title><?php wp_title(); ?></title>

in your <head>, under header.php?

UPDATE

Apparently a change was made to the way titles are handled as of 4.4. Here is a link that explains how to use the new code:

https://www.developersq.com/change-page-post-title-wordpress-4-4/

/*
 * Override default post/page title - example
 * @param array $title {
 *     The document title parts.
 *
 *     @type string $title   Title of the viewed page.
 *     @type string $page    Optional. Page number if paginated.
 *     @type string $tagline Optional. Site description when on home page.
 *     @type string $site    Optional. Site title when not on home page.
 * }
 *     @since WordPress 4.4
 *     @website: www.developersq.com
 *     @author: Aakash Dodiya
*/
add_filter('document_title_parts', 'dq_override_post_title', 10);
function dq_override_post_title($title){
   // change title for singular blog post
    if( is_singular( 'post' ) ){ 
        // change title parts here
        $title['title'] = 'EXAMPLE'; 
    $title['page'] = '2'; // optional
    $title['tagline'] = 'Home Of Genesis Themes'; // optional
        $title['site'] = 'DevelopersQ'; //optional
    }

    return $title; 
}

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