In PCB, when a copper layer is hidden while "rebuilding all" copper areas, the copper area assigned to the hidden layer merges with a visible layers. For example, in 2-layer, when Top layer is hidden from view (and Bottom layer is shown), "Rebuilding All" of Copper Areas fails to properly rebuild the Top layer copper area.   More importantly, in "Copper Area Management", the otherwise Top layer copper area is NOW assigned to Bottom layer.