Was your first flight after the promotion already in F, or in J on a 2-class aircraft? My humble experience: J to F opup on my 5th anniversary (as SL), and Y to J on my 10th (as GO). On my promotion from SL to GO, a long haul Y to J opup. Also was upgraded twice as a nobody.

OTOH, I share your experience on the futility of seat blocking. It seems that the seat block never works when my MPC number is in the booking, but I always have empty seats next to me when I fly as a nobody.

BTW, I enjoy reading your posts on CX and thanks for the relatively unbiased perspective!