You are correct that it is taking the average of all points when it should only be considering points that make up the convex hull. I would definitely submit a bug report, as I don’t see one currently on Unreal Issues. It should be a very simple fix.
In the meantime you can fix this in the C++ source for yourself if you are using it. It’s in KismetMathLibrary.cpp. If you are in blueprint only, you could write the algorithm yourself using the link below, although it will be pretty complex for BP. This is the same source that the Unreal implementation comes from, so be sure to correct its center as well.