Hi there, I already replied to your question by email, but in case anyone else may find my response useful:
... Oneworld (QF) has been known to occasionally show phantom
seats on IB flights that don't actually exist. Since your search
included Oneworld (BA) and BA didn't show anything, my guess would be
that those seats don't actually exist.
So, basically, that was a temporary QF site fluke on IB flights and I suggest using Oneworld (BA) for this particular trip/route in the future.