Example plugin to log the rewrite_rules to debug.log. WP_DEBUG_LOG needs to be enabled on the site.

1tw_log_rewrite_rules_array.php

<?php
/*
 * Plugin Name: rewrite_rules log plugin
 * Description: This plugin adds a log entry to debug.log each time permalinks are flushed.
 * Author: Tony Warwick
 * Version: 1.0.0
 */
 
// ==== Debug ====
if ( ! function_exists('ee_write_log')) {
   function ee_write_log ( $log )  {
      if ( is_array( $log ) || is_object( $log ) ) {
         error_log( print_r( $log, true ) );
      } else {
         error_log( $log );
      }
   }
}

// ==== Function to log rewrite_rule ====
function tw_log_rewrite_rules_array($rewrite_array)
{
    $e = new \Exception;
    ee_write_log(
        array(
            'time' => date("Y-m-d H:i:s"),
            'rewrite_array' => $rewrite_array,
            'stacktrace' => $e->getTraceAsString(),
            'EE-Check' => isset($rewrite_array['events/?$']) ? 'true' : 'false'
        )
    );
    return $rewrite_array;
}
add_filter('rewrite_rules_array', 'tw_log_rewrite_rules_array', 10, 1);

2tw_ee_log_rewrite_rules_on_404.php

<?php // Do not include the opening PHP tag if you alreayd have one.

// Use this function with the above to confirm what the rewrite rules are when a request returns 404
// This function checks if the current request URI contains 'event' to coverthe default events slug
// if you have change the slug you'll need to update it in the function.
function tw_ee_log_rewrite_rules_on_404() {
    // Hooking into wp_footer so we can uss is_404()
    if( is_404() ) {
        // So have a 404, lets checks its for an EE event using the global $wp->request
        global $wp;
        if( strpos($wp->request, 'event') !== false) {
        // Request contains 'event', pull the rewrite rules.
            $rules = get_option( 'rewrite_rules' );
            // Write the rules to the log.
            ee_write_log(
                array(
                    'rewrite rules on 404' => $rules,
                    'request' => $wp->request,
                )
            );
        }
    }
}
add_action('wp_footer', 'tw_ee_log_rewrite_rules_on_404');