diff --git a/src/wp-includes/block-supports/layout.php b/src/wp-includes/block-supports/layout.php
index 3a1f5e7a0598d..13b0eb5ebd970 100644
--- a/src/wp-includes/block-supports/layout.php
+++ b/src/wp-includes/block-supports/layout.php
@@ -1008,10 +1008,29 @@ function wp_render_layout_support_flag( $block_content, $block ) {
 	$first_chunk                 = $block['innerContent'][0] ?? null;
 	if ( is_string( $first_chunk ) && count( $block['innerContent'] ) > 1 ) {
 		$first_chunk_processor = new WP_HTML_Tag_Processor( $first_chunk );
-		while ( $first_chunk_processor->next_tag() ) {
-			$class_attribute = $first_chunk_processor->get_attribute( 'class' );
+		/*
+		 * Use a stack to track open elements as tags are visited. Void elements
+		 * (those without a matching closing tag) are excluded so they don't
+		 * accumulate on the stack. At the end of the chunk, every element still
+		 * on the stack is unclosed — meaning its closing tag lives in a later
+		 * innerContent entry alongside the inner blocks, which makes it the
+		 * inner-block container. Elements that open and close within this chunk
+		 * are siblings that precede the inner blocks and should be ignored.
+		 * The last unclosed element with a class attribute is the best candidate
+		 * for the inner-block wrapper.
+		 */
+		$tag_stack = array();
+		while ( $first_chunk_processor->next_tag( array( 'tag_closers' => 'visit' ) ) ) {
+			if ( $first_chunk_processor->is_tag_closer() ) {
+				array_pop( $tag_stack );
+			} elseif ( ! WP_HTML_Processor::is_void( $first_chunk_processor->get_tag() ) ) {
+				$tag_stack[] = $first_chunk_processor->get_attribute( 'class' );
+			}
+		}
+		foreach ( array_reverse( $tag_stack ) as $class_attribute ) {
 			if ( is_string( $class_attribute ) && ! empty( $class_attribute ) ) {
 				$inner_block_wrapper_classes = $class_attribute;
+				break;
 			}
 		}
 	}
diff --git a/tests/phpunit/tests/block-supports/layout.php b/tests/phpunit/tests/block-supports/layout.php
index f47a7a3e35d2b..08520085e39b3 100644
--- a/tests/phpunit/tests/block-supports/layout.php
+++ b/tests/phpunit/tests/block-supports/layout.php
@@ -354,6 +354,27 @@ public function data_layout_support_flag_renders_classnames_on_wrapper() {
 				),
 				'expected_output' => '<p>A paragraph</p>',
 			),
+			'outer wrapper targeted when sibling element precedes inner blocks' => array(
+				'args'            => array(
+					'block_content' => '<div class="wp-block-group"><div class="wp-block-group__header">Header</div><p>Inner block</p></div>',
+					'block'         => array(
+						'blockName'    => 'core/group',
+						'attrs'        => array(
+							'layout' => array(
+								'type' => 'default',
+							),
+						),
+						'innerBlocks'  => array(),
+						'innerHTML'    => '<div class="wp-block-group"><div class="wp-block-group__header">Header</div><p>Inner block</p></div>',
+						'innerContent' => array(
+							'<div class="wp-block-group"><div class="wp-block-group__header">Header</div>',
+							null,
+							'</div>',
+						),
+					),
+				),
+				'expected_output' => '<div class="wp-block-group is-layout-flow wp-block-group-is-layout-flow"><div class="wp-block-group__header">Header</div><p>Inner block</p></div>',
+			),
 		);
 	}