Frontend Ajax problem | WordPress.org

My plugin Wp Photo Album Plus uses Ajax in many instances, both backend and frontend.

Because one should not have admin-ajax.php in an url from the frontend, i use the redirect method as follows:

// Add rewrite rule
function wppa_ajax_rewrite_rule() {

	add_rewrite_rule( 'wppaajax/?$', 'index.php?wppa_ajax=1', 'top' );
	$rewrite_rules = wppa_get_option( 'rewrite_rules', array() );
	if ( ! is_array( $rewrite_rules ) || ! in_array( 'index.php?wppa_ajax=1', $rewrite_rules ) ) {
		flush_rewrite_rules();
	}
}
add_action( 'init', 'wppa_ajax_rewrite_rule' );

// Add wppa_ajax to query vars
function wppa_ajax_query_vars( $query_vars ) {

	$query_vars[] = 'wppa_ajax';
	return $query_vars;
}
add_filter( 'query_vars', 'wppa_ajax_query_vars' );

So, rather than https://betatest.opajaap.nl/wp-admin/admin-ajax.php we can have https://betatest.opajaap.nl/wppaajax/ as url.

This works almost always…. except:

when an other plugin assumes there is consustency in the execution of the admin_init hook and the function is_admin().

The following test plugin (2 files) demonstrates the issue:

Filename: demo-bug.php:

<?php
/*
 * Plugin Name: Demo bug
 * Description: Shows a bug in frontend ajax method
 * Version: 1
 * Author: J.N. Breetvelt a.k.a. OpaJaap
*/

if ( ! class_exists( 'DemoBugClass' ) ) {
	
	class DemoBugClass {
		
		function __construct() {
			
			$this->includes();
		}
		
		function includes() {
			
			if ( is_admin() ) {
						
				require_once( dirname( __FILE__ ) . '/demo-bug-functions.php' );
			
			}
		}
	}
	
	global $DenoBug;
	
	$DemoBug = new DemoBugClass(); 
}

add_action( 'admin_init', 'bug_test_function' );

Filename: demo-bug-functions.php:

<?php

function bug_test_function() {
	// Do nothing 
}

When this plugin is activated and we use the redirect ajax method, we get the following fatal error:

PHP Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function “bug_test_function” not found or invalid function name in /mnt/web208/b3/15/51893315/htdocs/opajaap/betatest/wp-includes/class-wp-hook.php:324

When this plugin is active and we use the admin-ajax.php method, it works withoput error.

So far, i noticed that a similar error is generated under these conditions by pluguns Disable Gutenberg and WPForms Lite. This is the reason for this topic.

these are my question:
1. Do i do something wrong, and if so how can i fix this?
2. Should not the conflicting plugins load the php files that containg the missing functions unconditionally? I proposed this to the Disable Gutenberg boys, bu they do not want to do that.
3. Is this a WP bug (is_admin() returns false because of not or too late defining WP_ADMIN )?