stubborn 4xx/5xx Error Handler

In previous versions of BIG-IP ASM, you'd need to disable ASM when sending a response from an iRule (Click here). I didn't think that would be necessary in 9.4.2+ with the plugin architecture. Can you open a case with F5 Support asking them to investigate this? If you want to try one more thing, you could explicitly disable the ASM plugin when sending a redirect:

when HTTP_REQUEST {
   set myhost [HTTP::host]
}
when HTTP_RESPONSE {
    Initialise a variable to track whether we're redirecting this request
   set redirect 0
   log local0. "[IP::client_addr]:[TCP::client_port]: Request $myhost generated response status: [HTTP::status]"
   if { ([HTTP::status] starts_with "4")} {
      log local0. "[IP::client_addr]:[TCP::client_port]: Redirecting to 4xx.htm"
      HTTP::redirect https://$myhost/images/errors/err_4xx.htm
      set redirect 1
   } elseif { ([HTTP::status] starts_with "5")} {
      log local0. "[IP::client_addr]:[TCP::client_port]: Redirecting to 5xx.htm"
      HTTP::redirect https://$myhost/images/errors/err_5xx.htm
      set redirect 1
   } else {
      log local0. "[IP::client_addr]:[TCP::client_port]: Doing nothing"
   }
}
when HTTP_CLASS_SELECTED {
    If the request is being redirected, disable ASM
   if {$redirect}{
       Enable ASM by default
      PLUGIN::disable ASM
   } else {
       Enable ASM by default
      PLUGIN::enable ASM
   }
}

Aaron