It looks like I finally found what the bug is. On bohemia, each time I was accepting the rewards, I was on my fort. This time, it's on Aquitaine, I was on my fortress when I accepted my 12 hours reward, and I recieved no diamonds. To make sure what I found was correct, I tested it with the 5 minutes reward, and the results were the same, I lost the diamond for the 5 minutes reward.

So the bug is: If you accept a reward that give diamonds (5 minures or 12 hours), make sure you are accepting the quest inside a city and not a fort or fortress. For some reasons, the reward doesn't work when not in a city, and there's nothing to prevent players about it.