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Can you share your metric SQL? If you're using GA4, you should just use

user_pseudo_id

in your metric SQL and in your Experiment Assignment Query (you can find this under your Datasources).

For others who are having this problem, we solved it by removing the GA1.1 from the beginning of the IDs with a simple string replace. We were not entirely sure why, but it seems that the id is different when firing the GA tracking callback from the server side,