Google Tag Manager not working after upgrading TYPO3 from v11 to v12 (code present, no tracking)

we recently upgraded a TYPO3 website from version 11 to version 12, and since the update, Google Tag Manager (GTM) tracking is no longer working properly.

Here’s what we’ve observed:

• GTM is integrated via the "Olli macht’s Cookie Manager" extension.
• The GTM code is present in the frontend source code.
• However, no pageviews are tracked in Google Analytics.
• Even if we embed the GTM manually, without using the Cookie Manager, it still doesn’t work.
• If we deploy the old TYPO3 v11 installation under a subdomain with the same GTM container, tracking works as expected.
• A basic static PHP page with the same GTM code also tracks pageviews correctly.
• Today we updated both TYPO3 v12 and the Cookie Manager extension to the latest versions, but the issue persists.

At this point, I’m a bit clueless. Has anyone else run into this issue after upgrading to TYPO3 v12? Are there any known issues with GTM in TYPO3 v12, or changes in how scripts are handled?

Any ideas or tips would be highly appreciated!

nvm om cookiemanager added a feature vor GTM Consent Mode V2 which was per default active, you can disable it in Typo3 Constantseditor.