I opened a bug report for this issue, and the response back from that was that I need to add set check_backend_singleton_instance from the BackendOptions to false as it is a problem with running Quill in the compiler explorer environment.

So, the starting code will look like this

  quill::BackendOptions backend_options;
  backend_options.check_backend_singleton_instance = false;   // needed for compiler explorer environment only
  quill::Backend::start(backend_options);

Here's a working compiler explorer environment to show you the full code.