Duplicated artboards containing Export Assets link to the new artwork not the original
As the title says, Assets in the Export panel move from the original artboard and become linked to the new artwork on the duplicated artboards. Subsequently when you remove any artwork from the new artboard, you inadvertantly remove the asset from the Export panel.
Duplicate an artboard (which contains an asset in the export assets panel) via the Alt-drag method. Ensure the move/copy artwork option is enabled. Delete the new copy of the artwork/asset on the new artboard, and the asset is removed from the export panel.
Any link to/connection to export assets should be retained by the original artwork on the original artboard, thereby enabling the new duplicated artwork to act as a guide for a modified creation and such that once any components are removed, the original asset remains intact.