I've been having the same problem, waiting on some TBS-LHR flights to open up. I'm downgraded to the Y cabin on a biz award. BA is blocking out all Saturday flights until November-ish. They seem to clear 7 days ahead, but haven't determined a pattern. Usually if it is showing I9 it clears, the day I am flying is I0

(but D9 J9 C9), 4/20 seats showing occupied on the seatmap.