Why TITLE tag is empty for some bbPress pages?

bbPress has own function that generates title used in HEAD TITLE tag. But, that is not working with new themes with title-tag feature support.

Until the bbPress core is updated to work with new method WordPress generates TITLE, this is a small piece of code that can fix this problem.

add_filter('document_title_parts', 'bbhelp_club_document_title_parts');
function bbhelp_club_document_title_parts($parts) {
    if (empty($parts['title']) && is_bbpress()) {
        $parts['title'] = trim(bbp_title('', ''));
    }

    return $parts;
}

You can add this code into the theme functions.php file.