Well, on the one hand, I do think the intention of the change is to be able to do “nasty” stuff like this.

On the other hand, in my opinion BA have botched this change and not yet really fully decided how to set it up.

If you use ba.com to try a dummy booking AGP-LAX, you may well find availability showing and I’m not sure why BA agents sometimes see different things to customers on the website. POC means that agents must enter individual sectors in the order in which they are flown. If they remove your DUB-LHR and then add an AGP-LHR then the POC logic programmed into Amadeus will not work correctly. So, this could be a system error, an agent error, or a desired outcome.

I’ve had similar issues last week with Lufthansa and in the end gave up. Availability was showing on the website and I could enter all details and only at the payment stage did I get a message that the flights were no longer available. In the end I just gave up and booked something else. I feel this is all getting too complex for airlines and reservation systems, let alone customers.