wordpress-website-lifecycle/mu-plugins/_core-http-reencode-query-string-spaces.php

<?php

/*
 * Plugin Name: Re-encode spaces in query string with %20
 */

add_action(
    'init',
    static function () {
        $parsed_url = parse_url($_SERVER['REQUEST_URI']);
        if (!isset($parsed_url['path']) || $parsed_url['path'] === '') {
            return;
        }

        $request_uri = $parsed_url['path'];
        $query_string = '';
        if (isset($parsed_url['query'])) {
            $query_parameters = [];
            parse_str($parsed_url['query'], $query_parameters);
            $query_string = http_build_query($query_parameters, '', '&', PHP_QUERY_RFC3986);
            $request_uri .= '?' . $query_string;
        }

        $_SERVER['REQUEST_URI'] = $request_uri;
        $_SERVER['QUERY_STRING'] = $query_string;
    },
    0,
    0
);