In the case of DDOS mitigation, the connections aren’t “missing”… they’re being outright closed, ignored, or rejected by the mitigation partner. All of which is already easily visible if they wish to monitor them.
It’s very easy for them to see when a connection just drops, as well. They don’t need an additional report to see this. They just need to look for sessions that ended abnormally (ie not “log off”). Even Alt+F4 will cleanly close the client, and the client can cleanly terminate the session.