They didn't know that at the time, did they? (well, River did)

Fridge Horror: Combine the new information that Timelords take a long
time to actually die with the relatively rapid burning of the 11th
Doctor's body in The Impossible Astronaut. "Don't cremate me" indeed!

Massive oversight by the Doctor/UNIT? The Master's original plan as stated was for the second cyber-rain to kill off the living humans, who then become cybermen. The Master died on Earth, presumably with this plan still in place. So the Master simply went to the Nethersphere, presumably with a backup plan in place for…

If that's the scene I'm thinking of, I was actually surprised at the quality of the practical effects (but only for that one shot). The victim's head had zero texture and the blood looked ultra thin/watery.