Index: src/wp-content/themes/twentyseventeen/assets/css/editor-blocks.css
===================================================================
--- src/wp-content/themes/twentyseventeen/assets/css/editor-blocks.css	(revision 58009)
+++ src/wp-content/themes/twentyseventeen/assets/css/editor-blocks.css	(working copy)
@@ -458,15 +458,17 @@
 	margin-top: 0.5em;
 }
 
-/* Caption styles*/
+/* Caption styles */
 
-[class^="wp-block-"]:not(.wp-block-gallery) figcaption {
+[class^="wp-block-"]:not(.wp-block-gallery) figcaption,
+[class*=" wp-block-"]:not(.wp-block-gallery) figcaption {
 	font-style: italic;
 	margin-bottom: 1.5em;
 	text-align: left;
 }
 
-.rtl [class^="wp-block-"]:not(.wp-block-gallery) figcaption {
+.rtl [class^="wp-block-"]:not(.wp-block-gallery) figcaption,
+.rtl [class*=" wp-block-"]:not(.wp-block-gallery) figcaption {
 	text-align: right;
 }