Enabling the x-forwarded-for setting does not populate the Source User column with x-forwarded-for information when IPv6 traffic is used. This is the expected behavior.
When the dataplane generates the URL filtering log, the 32-bit source User-ID field is used to carry the IPv4 address in the x-forwarded-for header.
The x-forwarded-for setting is enabled with the following command:
# set deviceconfig setting ctd x-forwarded-for yes