The aircon in T5 is using a system whereby cold air arrives at floor level and rises to the ceiling, without extensive fans and forced air movement. Such a setup is bad at handling hot spots like the arrays of chillers around the enclosed F lounge (the bars, the food area, etc - all over the space, since the bars are spread out). So I'm not surprised that the lounge is too hot while the terrace is cooler.

Even the rest of the terminal is usually too warm, even if the outside temperature is cold and the sun is not shining, but the F lounges are particularly prone to overheating.