@@ -1661,6 +1661,7 @@ public function sanitize() {
 							$implicit_block_type = str_starts_with( $block_type, 'core/' )
 								? substr( $block_type, /* 'core/' */ 5 )
 								: $block_type;
+							$make_implicit       = $block_type !== $implicit_block_type;
 
 							switch ( $block_processor->get_delimiter_type() ) {
 								// Track when blocks open and when they don’t self-close.
@@ -1691,6 +1692,7 @@ public function sanitize() {
 							// Filter block attributes for opening delimiters.
 							if ( $block_processor->opens_block() ) {
 								$original_attributes = $block_processor->allocate_and_return_parsed_attributes();
+								$voider              = WP_Block_Processor::VOID === $block_processor->get_delimiter_type() ? '/' : '';
 
 								if ( isset( $original_attributes ) ) {
 									$filtered_attributes = filter_block_kses_value(
@@ -1702,10 +1704,24 @@ public function sanitize() {
 
 									if ( $original_attributes !== $filtered_attributes ) {
 										$serialized_attributes = serialize_block_attributes( $filtered_attributes );
-										$voider                = WP_Block_Processor::VOID === $block_processor->get_delimiter_type() ? '/' : '';
 										$text                  = " wp:{$implicit_block_type} {$serialized_attributes} {$voider}";
+									} elseif ( $make_implicit ) {
+										$type_at    = strpos( $comment, 'wp:' ) + 3;
+										$after_type = $type_at + strlen( $block_type );
+										$close_at   = strpos( $comment, '-->' );
+
+										$text = (
+											substr( $comment, 0, $type_at ) .
+											$implicit_block_type .
+											substr( $comment, $after_type, $close_at - $after_type )
+										);
 									}
+								} elseif ( $make_implicit ) {
+									$text = " wp:{$implicit_block_type} {$voider}";
 								}
+							} elseif ( $make_implicit ) {
+								// Normalize closing block delimiters too.
+								$text = " /wp:{$implicit_block_type} ";
 							}
 						}