diff --git a/src/wp-includes/class-wp-theme.php b/src/wp-includes/class-wp-theme.php
index 6952c59bda494..f5b9f6bab238f 100644
--- a/src/wp-includes/class-wp-theme.php
+++ b/src/wp-includes/class-wp-theme.php
@@ -1338,13 +1338,21 @@ public function get_post_templates() {
 			$files = (array) $this->get_files( 'php', 1, true );
 
 			foreach ( $files as $file => $full_path ) {
-				if ( ! preg_match( '|Template Name:(.*)$|mi', file_get_contents( $full_path ), $header ) ) {
+				$headers = get_file_data(
+					$full_path,
+					array(
+						'Template Name'      => 'Template Name',
+						'Template Post Type'  => 'Template Post Type',
+					)
+				);
+
+				if ( ! $headers['Template Name'] ) {
 					continue;
 				}
 
 				$types = array( 'page' );
-				if ( preg_match( '|Template Post Type:(.*)$|mi', file_get_contents( $full_path ), $type ) ) {
-					$types = explode( ',', _cleanup_header_comment( $type[1] ) );
+				if ( $headers['Template Post Type'] ) {
+					$types = explode( ',', $headers['Template Post Type'] );
 				}
 
 				foreach ( $types as $type ) {
@@ -1353,28 +1361,28 @@ public function get_post_templates() {
 						$post_templates[ $type ] = array();
 					}
 
-					$post_templates[ $type ][ $file ] = _cleanup_header_comment( $header[1] );
+					$post_templates[ $type ][ $file ] = $headers['Template Name'];
 				}
 			}
 
-			$this->cache_add( 'post_templates', $post_templates );
-		}
+			if ( current_theme_supports( 'block-templates' ) ) {
+				$block_templates = get_block_templates( array(), 'wp_template' );
+				foreach ( get_post_types( array( 'public' => true ) ) as $type ) {
+					foreach ( $block_templates as $block_template ) {
+						if ( ! $block_template->is_custom ) {
+							continue;
+						}
 
-		if ( current_theme_supports( 'block-templates' ) ) {
-			$block_templates = get_block_templates( array(), 'wp_template' );
-			foreach ( get_post_types( array( 'public' => true ) ) as $type ) {
-				foreach ( $block_templates as $block_template ) {
-					if ( ! $block_template->is_custom ) {
-						continue;
-					}
+						if ( isset( $block_template->post_types ) && ! in_array( $type, $block_template->post_types, true ) ) {
+							continue;
+						}
 
-					if ( isset( $block_template->post_types ) && ! in_array( $type, $block_template->post_types, true ) ) {
-						continue;
+						$post_templates[ $type ][ $block_template->slug ] = $block_template->title;
 					}
-
-					$post_templates[ $type ][ $block_template->slug ] = $block_template->title;
 				}
 			}
+
+			$this->cache_add( 'post_templates', $post_templates );
 		}
 
 		if ( $this->load_textdomain() ) {
diff --git a/tests/phpunit/tests/admin/includesTheme.php b/tests/phpunit/tests/admin/includesTheme.php
index ed90cf9514ae5..499120d181042 100644
--- a/tests/phpunit/tests/admin/includesTheme.php
+++ b/tests/phpunit/tests/admin/includesTheme.php
@@ -222,6 +222,72 @@ public function test_get_post_templates_child_theme() {
 		);
 	}
 
+	/**
+	 * @ticket 42513
+	 *
+	 * @covers WP_Theme::get_post_templates
+	 */
+	public function test_get_post_templates_caches_results() {
+		$theme = wp_get_theme( 'page-templates' );
+		$this->assertNotEmpty( $theme );
+
+		switch_theme( $theme['Template'], $theme['Stylesheet'] );
+
+		// First call populates the cache.
+		$first_result = $theme->get_post_templates();
+		$this->assertNotEmpty( $first_result );
+
+		// Second call should return the same result from cache.
+		$second_result = $theme->get_post_templates();
+		$this->assertSame( $first_result, $second_result );
+	}
+
+	/**
+	 * @ticket 42513
+	 *
+	 * @covers WP_Theme::get_post_templates
+	 */
+	public function test_get_post_templates_uses_get_file_data() {
+		$theme = wp_get_theme( 'page-templates' );
+		$this->assertNotEmpty( $theme );
+
+		switch_theme( $theme['Template'], $theme['Stylesheet'] );
+
+		$post_templates = $theme->get_post_templates();
+
+		// Verify single-line header format is parsed correctly via get_file_data().
+		$this->assertArrayHasKey( 'page', $post_templates );
+		$this->assertArrayHasKey( 'template-header.php', $post_templates['page'] );
+		$this->assertSame( 'This Template Header Is On One Line', $post_templates['page']['template-header.php'] );
+	}
+
+	/**
+	 * @ticket 42513
+	 *
+	 * @covers WP_Theme::get_post_templates
+	 */
+	public function test_get_post_templates_cache_cleared_on_switch() {
+		$theme = wp_get_theme( 'page-templates' );
+		$this->assertNotEmpty( $theme );
+
+		switch_theme( $theme['Template'], $theme['Stylesheet'] );
+
+		// Populate cache.
+		$theme->get_post_templates();
+
+		// Clearing theme cache should require a fresh scan on next call.
+		wp_clean_themes_cache();
+
+		$child_theme = wp_get_theme( 'page-templates-child' );
+		switch_theme( $child_theme['Template'], $child_theme['Stylesheet'] );
+
+		$child_templates = $child_theme->get_post_templates();
+
+		// Child theme should include its own templates.
+		$this->assertArrayHasKey( 'foo', $child_templates );
+		$this->assertArrayHasKey( 'template-top-level-post-types-child.php', $child_templates['foo'] );
+	}
+
 	/**
 	 * Test that the list of theme features pulled from the WordPress.org API returns the expected data structure.
 	 *