Ok, after some investigation, I've found that the problem is that you need to use the UA ID on your Google tag's first line:

[...].googletagmanager.com/gtag/js?id=UA-XXXXXXXX-Y [...]

, as well as in the gtag('config'...) line. Otherwise, you will only get data in your new (GA4) property.

Hi, I've migrated two sites from UA to GA4. I've kept the gtag unchanged in both of them, which use the UA ID. The first one is showing data in both properties (UA and GA4). However the second one only shows data in the UA property (and only if I change the ID to the G- format will the GA4 property get data; but then the UA property won't). How can I get data in both? Maybe I need to include in the Google tag both "gtag('config', 'UA-XXXXXXXXX-Y');" and "gtag('config', 'G-XXXXXXXXX');" ?

Thank you for noticing! For us, this is a sign to think about updating this article)

Great list thanks for sharing, some of the links do not work any more but all good

Thanks!

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