Index: src/wp-content/themes/twentytwentyone/assets/sass/05-blocks/image/_style.scss
===================================================================
--- src/wp-content/themes/twentytwentyone/assets/sass/05-blocks/image/_style.scss	(revision 54423)
+++ src/wp-content/themes/twentytwentyone/assets/sass/05-blocks/image/_style.scss	(working copy)
@@ -57,6 +57,7 @@
 			@include media(mobile-only) {
 				margin-left: 0;
 				margin-right: 0;
+				float: none;
 			}
 		}
 	}