Setting 'debug_mode':true has no effect

Question

To investigate unrealistic realtime figures shown in GA (as described in this post), I enabled debug mode in the gtag configuration of our site:

<script async="" src="https://www.googletagmanager.com/gtag/js?id=G-12345ABCDE"></script>
<script>
      window.dataLayer = window.dataLayer || [];
      function gtag(){dataLayer.push(arguments);}
      gtag('js', new Date());
      gtag('config', 'G-12345ABCDE', { 'debug_mode':true });
</script>

As a result I would expect to see all incoming events in the Analytics DebugView. As GA realtime figures report up to 800 users in the last 30 minutes, there should be a mass of events showing up.

But the DebugView remains empty, nothing is shown there.

What am I missing?