Try deferring output buffer processing until shutdown hook

includes/class-amp-theme-support.php

@@ -1650,49 +1650,54 @@ public static function is_output_buffering() {
 	 */
 	public static function finish_output_buffering( $response ) {
 		self::$is_output_buffering = false;
+		register_shutdown_function(
+			function () use ( $response ) {
+				try {
+					$response = self::prepare_response( $response );
+				} catch ( Exception $exception ) {
+					$title   = __( 'Failed to prepare AMP page', 'amp' );
+					$message = __( 'A PHP error occurred while trying to prepare the AMP response. This may not be caused by the AMP plugin but by some other active plugin or the current theme. You will need to review the error details to determine the source of the error.', 'amp' );
+
+					$error_page = Services::get( 'dev_tools.error_page' );
+
+					$error_page
+							->with_title( $title )
+							->with_message( $message )
+							->with_exception( $exception )
+							->with_response_code( 500 );
+
+					// Add link to non-AMP version if not canonical.
+					if ( ! amp_is_canonical() ) {
+						$non_amp_url = amp_remove_paired_endpoint( amp_get_current_url() );
+
+						// Prevent user from being redirected back to AMP version.
+						if ( true === AMP_Options_Manager::get_option( Option::MOBILE_REDIRECT ) ) {
+							$non_amp_url = add_query_arg( QueryVar::NOAMP, QueryVar::NOAMP_MOBILE, $non_amp_url );
+						}
+
+						$error_page->with_back_link(
+							$non_amp_url,
+							__( 'Go to non-AMP version', 'amp' )
+						);
+					}
 
-		try {
-			$response = self::prepare_response( $response );
-		} catch ( Exception $exception ) {
-			$title   = __( 'Failed to prepare AMP page', 'amp' );
-			$message = __( 'A PHP error occurred while trying to prepare the AMP response. This may not be caused by the AMP plugin but by some other active plugin or the current theme. You will need to review the error details to determine the source of the error.', 'amp' );
-
-			$error_page = Services::get( 'dev_tools.error_page' );
-
-			$error_page
-				->with_title( $title )
-				->with_message( $message )
-				->with_exception( $exception )
-				->with_response_code( 500 );
-
-			// Add link to non-AMP version if not canonical.
-			if ( ! amp_is_canonical() ) {
-				$non_amp_url = amp_remove_paired_endpoint( amp_get_current_url() );
-
-				// Prevent user from being redirected back to AMP version.
-				if ( true === AMP_Options_Manager::get_option( Option::MOBILE_REDIRECT ) ) {
-					$non_amp_url = add_query_arg( QueryVar::NOAMP, QueryVar::NOAMP_MOBILE, $non_amp_url );
+					$response = $error_page->render();
 				}
 
-				$error_page->with_back_link(
-					$non_amp_url,
-					__( 'Go to non-AMP version', 'amp' )
-				);
-			}
-
-			$response = $error_page->render();
-		}
+				/**
+				 * Fires when server timings should be sent.
+				 *
+				 * This is immediately before the processed output buffer is sent to the client.
+				 *
+				 * @since 2.0
+				 * @internal
+				 */
+				do_action( 'amp_server_timing_send' );
 
-		/**
-		 * Fires when server timings should be sent.
-		 *
-		 * This is immediately before the processed output buffer is sent to the client.
-		 *
-		 * @since 2.0
-		 * @internal
-		 */
-		do_action( 'amp_server_timing_send' );
-		return $response;
+				echo $response; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
+			}
+		);
+		return '';
 	}
 
 	/**