As was mentioned here, Communist China has an earlier four-engined amphibious flying boat for maritime patrol and waterbombing, the Harbin SH-5, which (as far as I've heard) remains in service today with the 'North Sea Fleet' out of Qingdao, and so presumably cannot be a failed design. To my inexpert eye, the AVIC AG600 and the Harbin SH-5 appear very similar in conception, layout, dimensions, MTOW, and performance. The turboprops of both flying boats derive from the Soviet Ivchenko AI-20 of the 1950s. Harbin and AVIC are basically the same Party-owned company.
For anyone here with knowledge: why the all-new AG600? If amphibious flying boats are deemed needed, why didn't Beijing simply continue (or restart) low-rate SH-5 production, adding improvements to the design from time to time? The recent blurb about AG600 progress in Aviation Week and Space Technology doesn't engage with such questions.