Index: src/wp-includes/class-wp-theme-json.php
===================================================================
--- src/wp-includes/class-wp-theme-json.php	(revision 55878)
+++ src/wp-includes/class-wp-theme-json.php	(working copy)
@@ -791,8 +791,14 @@
 	protected static function append_to_selector( $selector, $to_append, $position = 'right' ) {
 		$new_selectors = array();
 		$selectors     = explode( ',', $selector );
-		foreach ( $selectors as $sel ) {
-			$new_selectors[] = 'right' === $position ? $sel . $to_append : $to_append . $sel;
+		if ( 'right' === $position ) {
+			foreach ( $selectors as $sel ) {
+				$new_selectors[] = $sel . $to_append;
+			}
+		} else {
+			foreach ( $selectors as $sel ) {
+				$new_selectors[] = $to_append . $sel;
+			}
 		}
 		return implode( ',', $new_selectors );
 	}