diff --git src/wp-includes/rest-api.php src/wp-includes/rest-api.php
index c397bdc116..ce6e2d7a23 100644
--- src/wp-includes/rest-api.php
+++ src/wp-includes/rest-api.php
@@ -744,7 +744,11 @@ function rest_filter_response_fields( $response, $server, $request ) {
 		$ref   = &$fields_as_keyed;
 		while ( count( $parts ) > 1 ) {
 			$next         = array_shift( $parts );
-			$ref[ $next ] = array();
+			if ( isset( $ref[ $next ] ) && true === $ref[ $next ] ) {
+				// Skip any sub-properties if their parent prop is already marked for inclusion.
+				break 2;
+			}
+			$ref[ $next ] = isset( $ref[ $next ] ) ? $ref[ $next ] : array();
 			$ref          = &$ref[ $next ];
 		}
 		$last         = array_shift( $parts );
diff --git tests/phpunit/tests/rest-api.php tests/phpunit/tests/rest-api.php
index 2b8310f168..4943280cc0 100644
--- tests/phpunit/tests/rest-api.php
+++ tests/phpunit/tests/rest-api.php
@@ -560,6 +560,102 @@ class Tests_REST_API extends WP_UnitTestCase {
 		);
 	}
 
+	/**
+	 * Ensure that specifying a single top-level key in _fields includes that field and all children.
+	 *
+	 * @ticket 48266
+	 */
+	public function test_rest_filter_response_fields_top_level_key() {
+		$response = new WP_REST_Response();
+
+		$response->set_data(
+			array(
+				'meta' => array(
+					'key1' => 1,
+					'key2' => 2,
+				),
+			)
+		);
+		$request = array(
+			'_fields' => 'meta',
+		);
+
+		$response = rest_filter_response_fields( $response, null, $request );
+		$this->assertEquals(
+			array(
+				'meta' => array(
+					'key1' => 1,
+					'key2' => 2,
+				),
+			),
+			$response->get_data()
+		);
+	}
+
+	/**
+	 * Ensure that specifying a single top-level key in _fields includes that field and all children.
+	 *
+	 * @ticket 48266
+	 */
+	public function test_rest_filter_response_fields_child_after_parent() {
+		$response = new WP_REST_Response();
+
+		$response->set_data(
+			array(
+				'meta' => array(
+					'key1' => 1,
+					'key2' => 2,
+				),
+			)
+		);
+		$request = array(
+			'_fields' => 'meta,meta.key1',
+		);
+
+		$response = rest_filter_response_fields( $response, null, $request );
+		$this->assertEquals(
+			array(
+				'meta' => array(
+					'key1' => 1,
+					'key2' => 2,
+				),
+			),
+			$response->get_data()
+		);
+	}
+
+	/**
+	 * Ensure that specifying two sibling properties in _fields causes both to be included.
+	 *
+	 * @ticket 48266
+	 */
+	public function test_rest_filter_response_fields_include_all_specified_siblings() {
+		$response = new WP_REST_Response();
+
+		$response->set_data(
+			array(
+				'meta' => array(
+					'key1' => 1,
+					'key2' => 2,
+				),
+			)
+		);
+		$request = array(
+			'_fields' => 'meta.key1,meta.key2',
+		);
+
+		$response = rest_filter_response_fields( $response, null, $request );
+		$this->assertEquals(
+			array(
+				'meta' => array(
+					'key1' => 1,
+					'key2' => 2,
+				),
+			),
+			$response->get_data()
+		);
+	}
+
 	/**
 	 * @ticket 42094
 	 */