It is not just a question of outbound/inbound, rather it is any failure to fly any segment in the order issued. Thus, on AAA-BBB-CCC-DDD, if you fail to fly BBB-CCC, your CCC-DDD will be cancelled as well.

There are indeed examples of situations where people have properly flown, the gate reader scanners or other aspects of the software have not properly recorded the flight as having been boarded and an individual's itinerary has been improperly cancelled.

The bottom line is that if your onward journey, whether it is a return or not, is not cancelled, that is due to an error at BA and does not form a particularly useful data point any more than various exceptions which BA has granted to individuals mean that it has a policy to that effect.