The Replace with Other trait is working fine (and can be edited) when applied directly to a piece or the RWO trait is copied from one piece to another.
However, if you copy an entire piece (like duplicating a card in a deck) and you try to edit the RWO trait on the copy, the bug pops up. To work around it, I’ve been copying the piece, removing the RWO trait from the copy, going back to the original piece, copying the RWO trait by itself and pasting it into the new copy.
Once I’ve saved the mod and reloaded it for editing, I get the error when I attempt to edit the RWO trait on any of the pieces. So, my experience in the preceding post was before the mod was saved and reloaded.