Store data in variable from HTTP_REQUEST to LB_FAILED

Understand that variables exists through the life of a single TCP connection. You can have many HTTP requests within a single TCP connection, and indeed you can have many HTTP requests before an LB_FAILED event happens. If you throw some logging in the HTTP request and LB_FAILED event to show what's happening and when, you may find that the variable is being set (incorrectly) by subsequent HTTP requests before the failing event happens.

when HTTP_REQUEST {
    ... code to derive value ...
    log local0. "value is $value"
}
when LB_FAILED {
    log local0. "in event"
    if { [info exists value] } {
        log local0. "value = $value"
    }
}