With SNAT Automap enabled Performance L4 VIP - How to know the real client IP address ?

To correlate, you could try logging the server-side source port — [serverside {TCP::local_port}] — at the same time as the client-side source IP — [clientside {IP::remote_addr}]. I believe you can do this all when SERVER_CONNECTED. Like this (untested):

when SERVER_CONNECTED {
  log local0.info "Client [clientside {IP::remote_addr}] connected to [IP::server_addr]:[serverside {TCP::remote_port}] from port [serverside {TCP::local_port}]"
}