Ok. I tried to see if there was a vertical crack, but I couldn't find anything. I further watched the overflow tube after the tank was full to see if the plastic clip (holding the refill tube) was drawing up water and over the overflow riser. I was able to track a tiny stream of water flowing down the inside of the riser from the plastic clip. I moved the clip to several different locations around the top of the riser and could see the tiny stream of water following the clip location. The drawdown in the tank will continue to about 5/8" below the brim of the overflow riser. The pictures show the water drawn up from the tank and the tiny stream flowing down inside of the riser overflow.
If I set my tank water level to 3/8" below the top of the overflow riser, then the siphon (or wicking action) caused by the plastic clip does not occur.
If the water level is set above the 3/8" mark, then either one of two things:
1) if water level is set very high (near the brim of the riser), then water will drop far enough to activate the water valve and the tank will refill on a regular basis (in my case a cycle of about every 30 minutes),
2) if water level is set 1/4" below the riser, then a drawdown will still occur and tank level will drop to 5/8" below the top of overflow riser but the refill water valve does not activate. My valve seems to need about a 1/2" drop before it activates. This situation does not waste water by continually topping off the tank, but the drop of the water level (caused by the clip siphoning water) takes it down to 5/8" below the riser. This produces a poor flush even for liquid waste.
I set my water level yesterday at 3/8" below the riser and no drawdown begins and the water level is held. The flush within the bowl is also acceptable. The AS Champion 4 does have a note within the tank that says to set the water level 1/2" below overflow top. The tank was factory set with the water level set to the very top of riser. You could actually see the surface tension of the water above the riser. I liked the flush results and just kept it high, but the clip for the refill tube causes drawdown in the tank. The good news is that it is not a gasket problem or a damaged flush tower that I have to replace.