it was ‘likely’ to have been missed/assumed locked (and the emphasis in on “likely”) as the latch may have been “felt” closed/locked and not visually checked.
Most engine cowlings locks / latches (most….not all) can be visually checked. If part of the system on the 319 is on the bottom of the engine, this is positioned badly for a pilot to check. Again, I am not familiar with BA ops or 319 walk round procedures but have based these comments on the information that the latch is not easily vivisble.
Gulfstream latches are visible to pilots / ground crews as are most (not all) engine cowlings latches on biz jets…