The first thing to do is to check in debug mode GA4 if all events are coming through the server. Most likely the problem is somewhere in the configuration, or maybe the problem is with the consent status, if you have one.
@giovanni_fabbrocino did you end up getting to the bottom of this? I have a server side and client side property running in parallel and am seeing a fairly large discrepancy in lead form conversions as well.
I can see the event is being sent to the server container and is appearing in the GA4 debug view during testing, but I haven’t been able to determine a cause for why this might be.
Using two web and server properties in parallel is bad practice, it will never work correctly. It also doesn’t make any sense.
The server container in case of GA4 acts just as a proxy, it redirects traffic further to GA without any changes to it.