In the tank, there is a small tube (maybe 3/8") coming from the valve assembly and just hangs down into the riser drain tube.
Lift that small tube (that's just hanging there), so that it sits higher in the riser drain tube. If there is any hardware holding this tube in place, adjust it so as to keep the tip of the small tube higher up... otherwise get creative here.
This may fix your problem.
Why? From my specific experience, this small tube constantly runs water during the flush process. After the flush process completes, I have seen this tube create a siphon that continued to slowly draw water out of the tank into the riser drain tube. By elevating the tip of this tube, the siphon is more easily broken.
You can confirm if this is the problem: with everything in place as it is now, get a flashlight and look down the riser drain tube... if the small tube is dripping water, then this is your problem.
If this is not the case, then keep in mind that there are only a limited number of ways for water to leave the tank. One of those means is where your problem resides.