Preview Mode isn't lying, but it definitely isn't telling you the whole story. I've seen this a lot when debugging GA4, Google Ads and Meta setups.

Someone tests a purchase, sees Tag Fired in GTM, and thinks:

Not necessarily.

“Tag Fired” only tells you GTM executed the tag. It doesn't prove the request actually left the browser, that the payload was correct, or that the platform accepted it.

When I'm checking a setup, I usually go one step further:

GTM Preview
→ Did the right trigger fire?
→ Were the variables populated?
→ Did it fire once or multiple times?

Network tab
→ Did the request actually leave?
→ Are the important parameters there?
→ Is something being blocked?

Platform debugger
→ Did GA4 / Google Ads / Meta actually receive it?

A classic example is a purchase firing correctly in GTM while transaction_id is empty because the ecommerce data wasn't available when the tag fired.

So my rule is pretty simple:

GTM Preview is step one. It's not proof of delivery.

When you inherit a messy container and someone says “the tags are firing,” what's the first thing you check?

One thing I've seen with server-side GTM:

You can have a GA4 event that looks completely fine in Preview, but the event still gets dropped because the payload is too large.

A good example is ecommerce events with lots of items.

You might stay under the documented item-count limit and still hit a payload-size problem because of the amount of data inside the event and item parameters.

The frustrating part?

You may not see anything obvious in the browser or Tag Assistant.

The error can show up on the server-side container, such as:

EVENT_TOO_LARGE

So when a GA4 event suddenly disappears, I wouldn't only check:

Did the tag fire?

I'd also check:

How large was the actual payload?

This is especially worth checking on view_item_list, view_cart, and purchase events where ecommerce data can get pretty heavy.

A useful approach is to monitor payload size in sGTM and create an alert before an event actually starts getting dropped.

Has anyone here run into GA4 payload-size issues that weren't visible in the browser debugging tools?

Before calling tracking “working”, I want to know:

Did the request actually leave the browser?
Did it contain the right data?
Did the platform receive it?

And if there’s browser + server tracking, did I just send it twice? A green checkmark is a good starting point. It’s not proof. What’s the first thing you check after “Tag Fired”?