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It seems like there might be a timing issue with when the ‘login’ event is getting fired and when the tags are being executed, especially if it works on your local system. It’s possible that for some reason the user_id is not present or available at the time the tag fires in certain scenarios.
One approach could be setting up a JavaScript variable in GTM to pull your user_id from wherever it is being stored right after login (e.g., localstorage, cookie, etc). You can then configure your GA tag to use this variable as a User-ID field. Adding a check to your logic to ensure the user_id is present and correctly linked before the tag fires can help avoid any race conditions as well.
If your user_id does not change after a user logs in, you could also benefit from using persistent data layer variable. This would allow GTM to remember the user_id value even if the page refreshes or a new page is loaded.
Hardcoding is generally a less maintainable solution, and might break any advantage of using GTM. However, it may be a quick fix if you are unable to solve the problem with the above methods and you could revisit a more scalable solution once things are up and running.
Remember that these are configuration changes, and therefore can take a while to propagate. So, it’s possible that delay plays a part in what’s being observed. Trying a different system — a highly reliable VPN or a different location test — also helps eliminate any external factors that could only be encountered in system-specific or geography-specific cases.
