As a data point I had 2 F and 1 J award seats on a QR A380 flight from LHR to DOH in November, booked 8 weeks ago. One in our party with an F ticket had to cancel so I wanted to cancel it and move the person in J to F. Cancelled the F and monitored BA and AA for it to come back into inventory. Kept refreshing for 10 minutes and seat didn't back so feared the worst. Called EXP line since I had nothing to lose and the agent found and booked the 1 F seat. When I cancelled the J seat, after confirming the F, it also did not come back into inventory.
Thanks for the data point 44C. I didnt realise QR is now(was!) offering F awards on the LHR route!