Currently a known issue with ACF and Gutenberg:
https://www.advancedcustomfields.com/resources/known-issues/
Solution
Write specialized JavaScript in functions.php or one of the include files that adds a click event on the publish button which will stop the page from saving when a validation error exists and gives the user an alert letting them know what to change.
add_action('admin_footer', 'update_fields');
function update_fields()
{
global $pagenow;
echo '<script>
jQuery(window).on("load", function() {;
';
//If homepage or foundation home page
//Check slider for anything that's not mp4 videos, give error and don't let submit
if ( ('post.php' === $pagenow && isset($_GET['post']) && 'page' === get_post_type($_GET['post']) && $_GET['post'] == 53) || ('post.php' === $pagenow && isset($_GET['post']) && 'page' === get_post_type($_GET['post']) && $_GET['post'] == 141)) {
if($_GET['post'] == 53){
$sliderName = "main_slider";
}else{
$sliderName = "main_slider_foundation";
}
echo '
// When click update, check that
// Checks if cat is selected when publish button is clicked
jQuery( "body" ).on( "click", ".editor-post-publish-button", function( e ) {
var slider = $("[data-name='.$sliderName.'] [data-name=video]");
var errors = false;
slider.each(function(i, obj) {
var filename = $(this).find("[data-name=filename]").text();
if (filename=="" || filename.indexOf(".mp4") !== -1) {
}else{
errors = true;
}
});
if(errors){
alert("Please only upload mp4 videos");
return false;
}
} );
';
}
echo '});
</script>';
}