I accidentally found an article that answers my query about the unwanted stevia taste. It explains that stevia has undesired licorice taste at relatively low concentrations and some consumers are able to perceived bitterness at more than 300 ppm. The stevia sugar alternative also known as Rebaudioside A has two parts. The first is the sugar moiety which is responsible for sweet taste. The second is the steviol tail, causes the prolonged sweetness sensation and eventually the undesirable licorice taste.
The Natural Taste Consulting (NTC), claimed that they have natural stevia improver. It can make its taste more closer to sugar without the undesirable taste. It has no thaumatin and or glycyrhhizin, artificial sweeteners with known issues. The product is named as Stevia Improver MH-1-140-0.
My wild guest. Their natural stevia improver molecule binds to steviol tail preventing it from attaching to tongue taste receptor.