get_the_post_thumbnail() title and alt attributes not displaying

Question

I am attempting to display 'alt' and 'title' attributes on a post thumbnail image, but the only attribute that actually displays as passed is 'class'. 'alt' and 'title' are missing. Any ideas what could be the cause?

$thumbnail_id = get_post_thumbnail_id(  );
$image_alt = get_post_meta($thumbnail_id, '_wp_attachment_image_alt', true);
echo $image_alt; //correct alt attribute text

1. echo get_the_post_thumbnail($post->ID, 'post-thumbnail', ['class' => 'classTest'] ); //works

2. echo get_the_post_thumbnail($post->ID, 'post-thumbnail', ['alt' => 'altTest'] ); //doesn't work

3. echo get_the_post_thumbnail($post->ID, 'post-thumbnail', ['title' => 'titleTest'] ); //doesn't work

---

1. <img width="940" height="150" src="https:/...imgsrc.jpg" class="classTest attachment-post-thumbnail size-post-thumbnail wp-post-image"> classTest added

2. <img width="940" height="150" src="https:/...imgsrc.jpg" class="attachment-post-thumbnail size-post-thumbnail wp-post-image"> altTest missing

3. <img width="940" height="150" src="https:/...imgsrc.jpg" class="attachment-post-thumbnail size-post-thumbnail wp-post-image"> titleTest missing

Answer 1

Turns out this was caused by the following custom theme filter in functions.php. Commenting out fixed the problem.

function image_alt_tags($content) {
    global $post;
    preg_match_all('/<img (.*?)\/>/', $content, $images);
    if (!is_null($images)) {
        foreach($images[1] as $index => $value) {
            if (!preg_match('/alt=/', $value)) {
                $new_img = str_replace('<img', '<img alt="' . get_the_title() . '"', $images[0][$index]);
                $content = str_replace($images[0][$index], $new_img, $content);
            }
        }
    }
    return $content;
}
add_filter('the_content', 'image_alt_tags', 99999);

add_filter('post_thumbnail_html', 'thumbnail_filter', 99, 5);

function thumbnail_filter($html, $post_id, $post_thumbnail_id, $size, $attr) {
   // you can alter the resulted HTML here
   $html = preg_replace(array('/alt=\".*?\"/', '/title=\".*?\"/'), '', $html);
   return $html;
}