Html title best practices and usage (wp_title, title-tag support, etc..)?

Do you have any SEO plugins installed? They usually fiddle with this logic. Having Yoast enabled, the pre_get_document_title does not work for me, instead you should use wpseo_title per their documentation.

Having said that, with SEO plugins disabled and add_theme_support('title-tag'); enabled, the pre_get_document_title filter works for me without any problem.

add_action('after_setup_theme', function () {
    add_theme_support('title-tag');
}, 20);

add_filter('pre_get_document_title', function ($title) {
    return 'ORO!';
});

Result is, that I see ORO! as the page title.