Yes, this happens all the time -- don't ever expect award seats to go back into award inventory. Rather ironically your booking/cancellation activity would have increased the chance of the award being retained for revenue sale as it generates interest in the route/date making QF up the probability of future sale in its computation. So you contributed the last little percent that caused the award to be "lost" QF is historically more aggressive at doing this than most other airlines due to high premium seat sales in past years.