Register teh newly created style variations.

src/wp-includes/rest-api/endpoints/class-wp-rest-global-styles-controller.php

@@ -263,6 +263,23 @@ protected function prepare_item_for_database( $request ) {
 			} elseif ( isset( $existing_config['styles'] ) ) {
 				$config['styles'] = $existing_config['styles'];
 			}
+
+			// If the incoming request is going to create a new variation
+			// that is not yet registered, we register it here.
+			// This is because the variations are registered on init,
+			// but we want this endpoint to return the new variation immediately:
+			// if we don't register it, it'll be stripped out of the response
+			// just in this request (subsequent ones will be ok).
+			// Take the variations defined in styles.blocks.variations from the incoming request
+			// that are not part of the $exsting_config.
+			if ( isset( $request['styles']['blocks']['variations'] ) ) {
+				$existing_variations = isset( $existing_config['styles']['blocks']['variations'] ) ? $existing_config['styles']['blocks']['variations'] : array();
+				$new_variations      = array_diff_key( $request['styles']['blocks']['variations'], $existing_variations );
+				if ( ! empty( $new_variations ) ) {
+					wp_register_block_style_variations_from_theme_json_data( $new_variations );
+				}
+			}
+
 			if ( isset( $request['settings'] ) ) {
 				$config['settings'] = $request['settings'];
 			} elseif ( isset( $existing_config['settings'] ) ) {