Why would a tongue not detect mild sweetness after eating foods with high sweetness?

This happens due to of the "desensitization" of sensory receptors on the sensory cells of your tongue. This phenomenon happens in all of our senses, where a strong and continuous stimulus desensitizes us to the similar stimulus, so that it is harder to detect. For instance, this is how you adjust to new smells, or why it is harder to hear after attending a loud concert. It is type of how your body is able to adapt to latest environments. The sensory cells of your sensory organs regulate and "desensitize" these receptors in various ways. For instance, sensory receptors on the cell surface can be deactivated or cells can internalize the receptors and degrade them, both of these events effectively decrease the number of functioning receptors on the cell surface and therefore reduce its ability to detect a stimulus