THIS IS NORMAL!!!

You need to use Custom definitions!

The values ​​from user properties are passed to the event only if the value is not equal to the previous one, so it turns out that if there are no changes, then user_properties is present only in the first event.

To multiply user_properties to all events when uploading to Bigquery, you need to add the desired fields from user_properties to Custom definitions

In GA4 console go to:

Admin -> Data Display -> Custom definitions -> Create custom definitions