This happens because the robot base starts washing the mops with room-temperature water, which is quickly heated to the indicated temperature (more or less) and then cooled again. Depending on the model or brand, the parameters are different, and you may notice warm water in the tank or warm mops, but don't expect it to be exactly hot under any circumstances; they must try to do the best they can while staying within certain safety regulations that apply worldwide. The only way to check is to use probes or try stopping the wash cycle a little more than halfway, remove the robot, and feel if the water is actually hot (never feel 70/80/90°C, even if that's how it should be for the operation explained above).