Hi Valentina, sorry for the delay! Did this issue eventually resolve itself? There is a known lag between GA4 and BQ syncing, and since GrowthBook pulls results from BQ, that syncing lag can sometimes cause this problem.
If you still need help, could you send me a screenshot of the following?
1. The Experiment Assignment Query (Left navbar --> Metrics and Data --> Data Sources --> Select the one in question --> Find the "Experiment Assignment Queries" section.
2. What you see in the Big Query data table for the experiment
3. What you see in GA4