Example using a demo layer:
The output is white, but the actual result should be colored. The request is oversampling the original data by 25 thousands times (original data at 0.5deg/px, request at 0,00002 deg/px), so I'd suspect some numerical issue when computing the requested grid geometry.
One funny note, on my computer I actually get a green box (which would be a correct response) against that request, but not on cloudsdi, which is also a master/2.12.x build.
So, maybe there is some contextual bit yet to be discovered about it (I've been messing with ROIGeometry recently, wondering if some of my experiments are in the classpath).