This behavior was noted with
Arthoria the Saber as demonstrated in
this replay.
The text for Legend reads that there must be a Rare card in hand except for self, but Arthoria is able to select a different card position to replace with itself. This triggers Legend as the card is both played and newly inserted into the hand.
We need to either correct Legend's functionality or reword the description to match the outcome in this case. (I haven't checked to see if there are other persistent Rare Legends presently in the game.)