Update August 8th 2016
Curveballs seem to work only by themself. They still don't stack consistently.
Curveball + Throw Bonus yields only the Throw Bonus:
Curveball by itself yields only the Curveball Bonus:
Update July 31st 2016
This seems to have been partially fixed. I am consistently getting 10XP bonuses for every curveball. However.. it seems now if you throw a curveball with a Nice! Great! or Excellent! throw you only get the curveball 10XP. Seems Niantic just can't get it right, can they?
This seems to be exclusively a game problem.
Once the Pokéball is spinning as I showed in my question, that constitutes the 10XP Curveball bonus. However, according to a few people in the comments, the curve ball bonus gets awarded with normal throws as well.
Over the past couple days, I kept my eye closely on my bonuses and noticed I did get the 10XP Curve ball bonus once or twice, proving that the bonus is still in the game, however broken — completely.
I have yet to get the Curveball Bonus along with a Nice/Great/Excellent throw bonus, but until proven otherwise this has been shown to be in the game.