I have been trying to figure out what was going on with this bug, but mostly I have just not been playing my Ready and Waiting deck.
I had suspected that there might have been a change, and if there were any potential damage to you that might eliminate mastery, you only got a regular win. That cannot be the case, though
I just played a game against Neptune where I ended my turn with 100 life. He had only creatures with negative attack, some of them vampired, and a frozen weapon. Nothing that could conceivably deal me damage. He did nothing but play pillars (or maybe pendula), then ended his turn and died from his creatures damaging him. I got 61 gold plus some from the spin.
In this case, the bug has nothing to do with the timing of my healing or when the game ended, so when Mastery is checked for is irrelevant. If mastery is checked for at any point, I should gain mastery. I didn't heal. I just didn't take any damage, started at 100, ended at 100, was at 100 the entire time, and didn't get mastery.
I did spin a Shockwave, which is wonderful consolation for the missing 59 gold, but a bug is a bug, and this one is very annoying in that it significantly reduces my enjoyment of one of my favorite decks. I find myself instead playing a different deck against the Half-Bloods more often because it is more fun to play, even though I don't get so many upped cards from spins. Being frustrated when I lose because of a bad draw is one thing, but if I am frustrated when I win, even if I spin something, then I am just going to be frustrated all the time, and might as well do something more enjoyable.
Someone should figure out a way to check whether an opponent killing himself by damage during his own turn in other ways (say bolting himself or damaging your Voodoo Doll) also deprives you of mastery since the 1.2.9.3 update. An opponent losing during his own turn by decking out still gives mastery, I think. I ended a game against Lionheart by deckout with 500/500 life, (Thank you, Oracle), and got all the coin I was entitled to.