How can I set a pool and member from within an iRule?

I'm having trouble choosing a pool from an iRule. I've tried many combinations. Here's one, stripped down but with some debugging, that I think should work but doesn't:

when HTTP_REQUEST {
  if { [HTTP::uri] starts_with "/test/" } {
    pool testpool member 10.10.10.10 443
    log local0. "set pool to testpool, member to 10.10.10.10 443"
  }
}
when LB_SELECTED {
  log local0. "server [LB::server addr] pool [LB::server pool]"
}
when LB_FAILED {
  log local0. "server [LB::server addr] pool [LB::server pool]"
}
when SERVER_CONNECTED {
  log local0. "server [LB::server addr] pool [LB::server pool]"
}

The log message from within the URI match appears, but every log message after that shows the default pool and one of the default pool members.