Why does boiling water make dough taste sweet?
Flour is mostly starch, and starches are long chains of sugar molecules. When you add hot water to starches they gelatinize and burst, and these gelatinized starches soften the dough. Gelatinization works faster at higher temperatures.
There are enzymes in the flour which break the starch down into sugars, and they work more efficiently at high than room temperature as well, say 65°C.
Both these processes work at lower temperatures, but they work faster a higher temperatures, which is why adding boiling water gave you that result. At room temperature gelatinization works faster than enzymes action breaking starch down into sugars, so letting the dough rest will soften the dough without making it much sweeter.