Custom block problem

I am a plugin developer and i made several custom blocs for the gutenberg editor.
Everything works correctly, including the preview in the editor, until the published page is re-opened in the editor: I get the error message:

This page contains unexpected or invalid content and the two buttons ‘Resolve’ and ‘Convert to HTML’

The block definition of one of my custom blocks is:

function wppa_gutenberg_photo_block() {

    wp_register_script(
        'wppa-gutenberg-photo',
        plugins_url( 'js/wppa-gutenberg-photo.js', __FILE__ ),
        array( 'wp-blocks', 'wp-element' )
    );

    register_block_type( 'wppa/gutenberg-photo', array(
        'editor_script' => 'wppa-gutenberg-photo',
    ) );
}
add_action( 'init', 'wppa_gutenberg_photo_block' );

The content of the page as seen by the classic editor is:

<!-- wp:wppa/gutenberg-photo -->
<p>[photo 102]</p>
<!-- /wp:wppa/gutenberg-photo -->

It looks as if Gutenberg suddely forgot that my blocktype wppa/gutenberg-photo exists…

I now added this code, to ‘manually’ change wppa/gutenberg-photo into wp:shortcode as if it were a wp shortcode:

// Fix Gutenberg bug
function wppa_fix_gutenberg_shortcodes() {
global $wpdb;

	if ( strpos( $_SERVER['REQUEST_URI'], '/wp-admin/post.php' ) !== false &&
		 strpos( $_SERVER['REQUEST_URI'], 'action=edit' ) !== false ) {

		$post = strval( intval ( $_GET['post'] ) );

		// Get posts with wppa block_categories
		$posts = $wpdb->get_results( $wpdb->prepare( "SELECT * FROM $wpdb->posts
													  WHERE ID = %d
													  AND ( post_content LIKE %s
													  OR post_content LIKE %s )",
													  $post,
													  '%' . $wpdb->esc_like( 'wp:wppa/gutenberg-photo' ) . '%',
													  '%' . $wpdb->esc_like( 'wp:wppa/gutenberg-wppa' ) . '%' ), ARRAY_A );

		if ( ! empty( $posts ) ) {
			foreach( $posts as $post ) {
				$new_content = str_replace( array( 'wp:wppa/gutenberg-photo', 'wp:wppa/gutenberg-wppa' ), 'wp:shortcode', $post['post_content'] );
				$wpdb->query( $wpdb->prepare( "UPDATE $wpdb->posts
											   SET post_content = %s
											   WHERE ID = %d", $new_content, $post['ID'] ) );
			}
		}
	}
}
add_action( 'admin_init', 'wppa_fix_gutenberg_shortcodes' );

Now the error disappears.
This is obviously a workaround. I tried to declare my blocktype as shortcode but that conflicts with the generic wp shortcode blocktype.

What am i doing wrong?