SNAT is not working in a one-arm configuration.

Could you add the follow irule to the virtual server?

when HTTP_REQUEST {
   # Disable the stream filter for client requests
   STREAM::disable
}
when HTTP_RESPONSE {
    # Disable the stream filter for server responses
    STREAM::disable
# Enable the stream filter for text responses only

 if {([HTTP::status] == 200) && ([HTTP::header value Content-Type] starts_with "text")} {
  # Replace '10.100.100.200' with '10.200.6.1'
  STREAM::expression {@10.100.100.200@10.200.6.1@}
  # Enable the stream filter
  STREAM::enable
 }
}