Issue also reported by this user: https://growthbookusers.slack.com/archives/C01T6Q0TEG3/p1709290052964799?thread_ts=1709208338.521229&cid=C01T6Q0TEG3

Implementation

<script data-client-key="XXXXXXXXXXX" src="<https://cdn.jsdelivr.net/npm/@growthbook/growthbook/dist/bundles/auto.min.js>" async="async"></script>

Potentially this is the issue from auto.min.js

// GA4 (gtag and GTM options)
      window.gtag ? window.gtag("event", "experiment_viewed", p) : window.dataLayer && window.dataLayer.push({
        event: "experiment_viewed",
        ...p
      });

It's assuming that if gtag is present that's used and array sent instead of object via google tag manager think panic

That could be clearer in documentation for newer users, but I think ideally that's a preference you should be able to set potentially.

As mentioned in my original message for now i've added and have had success adding this before my auto.min.js implementation:

<script>
window.growthbook_config = window.growthbook_config || {};
window.growthbook_config.trackingCallback = (experiment, result) => {
          dataLayer.push({
            'event': 'experiment_viewed',
            'experiment_id': experiment.key,
            'variation_id': result.variationId
          });
};
</script>

My Ga4 configuration is implemented via a google tag if that helps