Christopher,
I think you are right, the reddish area is characteristic of a joint made by the furnace braze process which is used in many controlled production operations.
I think you had to drift the hub deeper onto the shaft and if force to move it was applied to the fan mounting flange instead of the hub it could have started the failure of the brazed joint.
I probably wouldn't have noticed the two piece pulley/hub and done the same thing because the hub is so narrow.
