My merc has an Eth-bugged Fortitude, imported from LOD to D2R. Anytime the Hero Editor reads it, it gets its defense reduced. Even simply importing the character file, saving it, and loading it in game, with no alterations or viewing the item on-screen will result in the lost defense.
Be sure to keep backs-ups and transfer the old fashioned way. I assume the same will happen to an E-bug Exile from LOD.
Yeah, this is not a bug. The item is bugged, and the editor "fixes" the bug. While I know it's not wanted, I can't change the whole defense calculation to support a bugged item. Best to just keep it in your shared stash when editing.