diff -Naur wordpress/wp-admin/includes/post.php wordpress/wp-admin/includes/post.php
--- wordpress/wp-admin/includes/post.php	2018-09-08 12:12:18.881200110 +0530
+++ wordpress/wp-admin/includes/post.php	2018-09-08 11:44:19.575396000 +0530
@@ -567,6 +567,8 @@
 		if ( isset( $post_data['post_format'] ) ) {
 			set_post_format( $post_ID, $post_data['post_format'] );
 			unset( $post_data['tax_input']['post_format'] );
+		} else {
+			unset( $post_data['tax_input']['post_format'] );
 		}
 
 		$updated[] = wp_update_post( $post_data );