iRules for Individual Testing of Farm Servers

Yes, we know about this issue. CR60667 - persistence doesn't work when directly selecting pool members.

You can work around it by setting a variable when you directly select the pool member (maybe something like "set used_pool_member 1") and then add another event to add the persistence record when that variable is set:

when SERVER_CONNECTED {
   if { [info exists used_pool_member] && $used_pool_member } {
      persist add source_addr
   }
}

Just be sure to use whatever persistence type you had configured (you didn't say whether it was source address or something else).