Resolution:
If you look at TIBCO® Object Service Broker for z/OS, Installing and Operating Software Release 5.0, page 75  Chapter 5, section  Installing the Service Gateway for CICS, you will see the following:

"...In a Terminal Owning Region (TOR), where the TIBCO Object Service Broker session is running in another CICS system, an internal HOPT transaction starts locally. This transaction then starts an HCUS CANCELUSER in the remote CICS system running TIBCO Object Service Broker. The extra processing is due to CICS function shipping restrictions within the DFHZNEP environment."

Therefore you only need to define HOPT in the TOR regions associated with Object Service Broker and not in the AOR itself. This is due to an IBM restriction on DFHZNEP processing whereby function shipping to another region is disallowed. Therefore for lost sessions, we START HOPT from DFHZNEP in the TOR and then let HOPT function ship a cancel user request to the AOR where HCUS is scheduled and runs. If you define the transaction in the AOR, it won't do any harm but it won't be used either.