Rules FAQ

• Highgarden Treasury can only react to cards that explicitly “remove” a character from a challenge via their text. Any other game rules that indirectly cause a character to no longer be participating in a challenge will not trigger Highgarden Treasury. (Source.)

• If an ability removes multiple characters from a challenge at the same time (for instance, A Wall of Roses), each removed character provides its own triggering condition for Highgarden Treasury.