Just an update.
The bug is still there after the 9/28 patch.
I still do not always get a reward for practice matches.
Again, this happens at times I should gain a reward (i.e. I have not yet met my daily limit).
I've noticed that happening to me once or twice as well. The first time I thought I wasn't paying attention, the second time I started feeling it was suspicious.
I believe it happens when you win before turn 6 (I won on turn 5 the second time it happened to me). Hopefully someone can double check. It's kinda hard for me as the deck I use in practice mode isn't made to win in 5 turns so it was a fluke that I got a mega nice starting hand and the opponent got nothing. Been trying to reproduce it for 5 days now, no luck.
I think it may be if you play one deck too many times in a row.
I have been changing decks more often and it seems to help in the last day or so. Too early to tell though.
Apparently its not the number of times you play a deck, it happened on the second attempt with one deck.
It happened again today (happens pretty much every day that I practice).
I won the game on turn 6 the time I failed to get a reward.
Subsequently, I checked the turn when I won. The earliest time I won after that was on my opponent's turn 6 ... he killed himself.
So as long as your opponent starts their turn 6 one does not have to worry about not getting gold due to finishing the game too quickly.