Disable plugins on cron and ajax page

I experienced a similar problem with an Ajax request. There are a couple of important things to note.

Allow me to first state the obvious:

Make sure that your code using the option_active_plugins filter is inside your mu-plugins folder. It will not work for if your plugin is a regular plugin. Not even if you name it aaaa-first-plugin.

Now for the problem I faced:

I had written my function like this:

add_filter( 'option_active_plugins', function ( $wp_enabled_plugins ) {

    // Quit immediately if in admin area.
    if ( is_admin() ) {
      return $wp_enabled_plugins;
    }

    // do other stuff

});

The problem is that when doing an AJAX call, is_admin() returns true. So I changed it to:

if ( is_admin() && !DOING_AJAX ) { ... }

Everything works fine for me now. Hope it helps someone.