The problem: if you add ALT tags after adding images to a page, WordPress doesn't by default use these (WTF!) - add this to functions to sort...

WordPress ALT tags.php

<?php

// Sorting out image ALTs
// from https://www.billerickson.net/code/wordpress-image-automatic-alt-text/

add_filter('render_block',

	function ($content, $block) {
		if ('core/image' !== $block['blockName']) {
			return $content;
		}

		if (isset($block) && isset($block['attrs']) && isset($block['attrs']['id'])) {
  		$alt = get_post_meta($block['attrs']['id'], '_wp_attachment_image_alt', true);
		} else {
  		$alt = '';
		}
		
		if (empty($alt)) {
			return $content;
		}

		// Empty alt
		if (false !== strpos($content, 'alt=""')) {
			$content = str_replace('alt=""', 'alt="' . $alt . '"', $content);

			// No alt
		} elseif (false === strpos($content, 'alt="')) {
			$content = str_replace('src="', 'alt="' . $alt . '" src="', $content);
		}

		return $content;
	},
	10,
	2
);

?>