Sorry page with LTM+APM

Are you using the linked iRule exactly, or have you modified it? As it stands the [active members ] check and HTTP::respond commands in the HTTP_REQUEST event should completely preempt the access policy. At the very least, if you still wanted to go through the access policy before showing the maintenance page, you could change HTTP_REQUEST to ACCESS_ACL_ALLOWED, and HTTP::respond to ACCESS::respond. The ACCESS_ACL_ALLOWED event and ACCESS::respond command are relatively synonymous with the respective HTTP event/commands but are guaranteed to fire after the policy has completed.