I finally got a reply that made sense (sort of) about why my flight did not get the OneStop service:
Thank you for your patience while we reviewed this case.
We have contacted our Operations teams and they have confirmed that there is a ‘One Stop Security’ trial currently in place in T3, however, each qualifying flight needs to meet certain criteria to be in scope for the trial. Your flight did not met the required criteria on this occasion and therefore all passengers did need to proceed through the standard process. We appreciate that this may not be the answer you were expecting, however as this process is in a trial stage, not all flights will be subject to the process.
We hope you find this information useful.