Since most leaking photos are taken from the ground by fans when the aircraft were in the air, i don't think "new aircraft type being developed at SAC" a necessary condition for witnessing the aircraft. The developers or testers of the aircraft were never the key leakers.
Chengdu and Shenyang are both mega cities with 10m population. When the new aircraft was takeing off or landing, there would be many people see it flying in low altitude. If they takes photos for the low altitude aircraft, it won't be low-res and blurry like the ones FC-31 leaked by now.
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As for this pic, I think it's existence is suspicious and inexplainable.
Where was this photo taken from? In another pkane? It's impossible for the testers to leak such highly classified information of a new plane.
If it was taken from a nearby building, then the aircraft must be in a side maneuvering in the low altitude and with the landing gear down. That wasn't the usual operation a new aircraft would be supposed to take.