Niantic said this: "Trainers, we've noticed a brief interruption in which Defender bonuses are not being awarded. Stay tuned for a fix shortly."
So unfortunately we're just going to have to wait. There's theories that they have disabled it to stop people cheating to get more coins but no clue when we're going get our full coins back :(
Just maybe take a day or two break from gymming. Or you can always level up your gym badge so you can make it stronger and get more item rewards from gyms! So there's still a benefit to gyms... it's just not as good.
