This means that you are not sending the same client_id or session_id that the client had on the web events.
You need to check that you are sending these parameters correctly, you may not have formatted the values of the cookies from where you get this data correctly.
This is similar to the _ga cookie value you use for client_id. When server side tracking GA4 uses FPID cookie for client_id. So you need to use the value from there.
If I remember correctly - the hash without numbers at the end is used, but you can check how it is sent in web tracking in the outgoing GA request from the server container.
Session id is stored in the cookie ga_{your measurement id}