Prevent already migrated blocks re-migrating

packages/block-library/src/media-text/deprecated.js

@@ -346,8 +346,20 @@ const v6 = {
 		);
 	},
 	migrate: migrateDefaultAlign,
-	isEligible( attributes ) {
-		return attributes.align === undefined;
+	isEligible( attributes, innerBlocks, { block } ) {
+		const { attributes: finalizedAttributes } = block;
+		// When the align attribute is switched to default to none, valid
+		// block markup wouldn't contain any alignment CSS class. Unfortunately,
+		// this deprecation's version of the block won't be invalidated due to
+		// the alignwide class still being in the markup. That is because the
+		// custom CSS classname support picks it up and adds it to the className
+		// attribute. At the time of parsing, the className attribute won't
+		// contain the alignwide class, hence the need to check the finalized
+		// block attributes.
+		return (
+			attributes.align === undefined &&
+			!! finalizedAttributes.className?.includes( 'alignwide' )
+		);
 	},
 };