Aha, found something else - I'd assumed that only the difference in standard state enthalpy between liquid and vapor matters, so I set water-liquid as 0 and water-vapor as 4.39 E+7 J/kgmol. However, when I revert them back to their default Fluent values (vapor = -2.418 E+8, liquid = -2.858 E+8), the steam becomes hotter, and the liquid becomes cooler - see image below.

This definitely resolves why I was not seeing evaporative cooling. However, I still don't believe the steam should be heating up - shouldn't it have the same temperature as the liquid, as all energy input is used to overcome the vaporization enthalpy?