Index: src/wp-includes/blocks.php
===================================================================
--- src/wp-includes/blocks.php	(revision 55512)
+++ src/wp-includes/blocks.php	(working copy)
@@ -260,7 +260,11 @@
 	if ( $has_style_file ) {
 		wp_style_add_data( $style_handle, 'path', $style_path_norm );
 
-		$rtl_file = str_replace( "{$suffix}.css", "-rtl{$suffix}.css", $style_path_norm );
+		if ( $is_core_block ) {
+			$rtl_file = str_replace( "{$suffix}.css", "-rtl{$suffix}.css", $style_path_norm );
+		} else {
+			$rtl_file = str_replace( ".css", "-rtl.css", $style_path_norm );
+		}
 
 		if ( is_rtl() && file_exists( $rtl_file ) ) {
 			wp_style_add_data( $style_handle, 'rtl', 'replace' );