Thanks for providing the dates and flights- I had a quick look at these while on the move yesterday, but then when I looked at again it seemed to have been “fixed”. So my best guess is that someone bought a few business seats on the flight which consumed the inflexible buckets fully and that at some point later a batch job released more.

See ExpertFlyer screenshot below. I’m not super familiar with QR fare codes but there’s at least R bucket availability on the BA code shares.