Found this thread after I had this issue, and I just sharing my experience.

Found the seat (and confirmed via chat that it was in fact not phantom, which had been the case earlier in the year for the very same seat/departure)
Failed upon payment -- in fact when the automated BAEC -> QRPC Avios transaction took place
Chat agent found my booking after I shared my details
Chat agent released the seat, and I could book it myself on the second try (having already manually transferred the Avios from BA to QR -- which I will do moving forward to avoid issues)