What am I doing wrong that is making my dishes spicier when served as leftovers. Am I not simmering long enough?
"Capsaicin is soluble in alcohol, but very little in water. Because of the burning sensation caused by capsaicin when it comes in contact with mucous membranes, it is commonly used in food products to give them added spice or "heat" (piquancy)."

Time was your magic ingredient. More capsaicin became soluble. I find rich dishes have no surprise punch reheated:

"Food that's high in oil or fat dissolves the capsaicin, so it can't continue to bind heat receptors. ... Full fat sour cream or ice cream."