If memory serves, a player can avoid learning the motif by placing the mouse cursor on the piece, the press the Right Mouse Button to display the context menu. It has an option to place the item in the Trade Window if the Trade UI is open. Also, perhaps the player can add the piece to the Trade Window by using a single click of the Left Mouse Button instead of double-click, but I don't recall whether I've tried to do that.So I went to trade some motifs with someone today and not having used the trade UI, wasn't aware of this bug.
The UI opens and naturally what do you - you double click on what you want to trade..... except no, that makes you learn the motif.
No player wants to learn the motif from the trade window, truly. I guarantee you this.