When attaching a large HTTP header to a response the policy execution finishes but it is never sent. SSG logs contain a "com.l7tech.server.message: Message was not processed: Undefined (-1)" along with a java.lang.ArrayIndexOutOfBoundsException exception.
The underling HTTP engine used by the Gateway has a default header size of 8K, anything over this size will cause these errors.
Release: L7SGA299000-8.4-API Gateway SOA Gateway-HARDWARE APPLIANCE DUAL CPU
This value can be increased for a particular listen port by setting advanced property maxHttpHeaderSize to the listen port. See attached listenPortAdvancedProperty.png image illustrating how to set the max header size to 16K.