Index: src/wp-content/themes/twentynineteen/style-editor.css
===================================================================
--- src/wp-content/themes/twentynineteen/style-editor.css	(revision 53789)
+++ src/wp-content/themes/twentynineteen/style-editor.css	(working copy)
@@ -602,6 +602,7 @@
 body .wp-block[data-align="center"],
 body .wp-block.aligncenter {
   margin-left: 0;
+  text-align: center;
 }
 
 @media only screen and (min-width: 768px) {
Index: src/wp-content/themes/twentynineteen/style-rtl.css
===================================================================
--- src/wp-content/themes/twentynineteen/style-rtl.css	(revision 53789)
+++ src/wp-content/themes/twentynineteen/style-rtl.css	(working copy)
@@ -5404,6 +5404,7 @@
   .entry .entry-content > .wp-block-group > .wp-block-group__inner-container > *.aligncenter {
     margin-right: 0;
     margin-left: 0;
+    text-align: center;
   }
 }
 
Index: src/wp-content/themes/twentynineteen/style.css
===================================================================
--- src/wp-content/themes/twentynineteen/style.css	(revision 53789)
+++ src/wp-content/themes/twentynineteen/style.css	(working copy)
@@ -5416,6 +5416,7 @@
   .entry .entry-content > .wp-block-group > .wp-block-group__inner-container > *.aligncenter {
     margin-left: 0;
     margin-right: 0;
+    text-align: center;
   }
 }