This article provides the information on traffic logs which shows " username="N/A" ".

In certain situations, the user uses security policies to log the remote VPN traffic logs. Username="N/A" will appear in the traffic information. The question "why username is N/A instead of my remote VPN username?" may come up for the user.

It is expected for the logs to not show username.

Username will be displayed in the security policy if the user has configured user authentications such "Pass-Through Authentication" or "captive portal authentication/ web-authentication" . For normal traffic it will be "N/A" - not applicable.

You can refer the below documents to know more about "Pass-Through Authentication" or "captive portal authentication/ web-authentication"

SRX Firewall Users
Configure Authentication Methods For SRX Firewall Users
User Role Firewall Security Policies