SEOUL — The couple saw brushes and paint cans in front of a paint-splattered canvas at a gallery in a Seoul shopping mall. So they added a few brush strokes, assuming it was a participatory mural.

Not quite: The painting was a finished work by an American artist whose abstract aesthetic riffs on street art. The piece is worth more than $400,000, according to the organizers of the exhibition that featured the painting.

Now it’s hard to tell where the artist’s work ends and the vandalism begins. “Graffitied graffiti,” a local newspaper headline said last week.

Either way, the piece, “Untitled,” by John Andrew Perello, the graffiti artist known as JonOne, is now a magnet for selfies. And on social media, South Koreans are debating what the vandalism illustrates about art, authorship and authenticity.

The artwork is displayed with paint cans, brushes and shoes that the artist used when he worked on it, one of the exhibition’s organizers, Kang Wook, said in an interview. He added, “There were guidelines and a notice, but the couple did not pay attention.”

Some social media users have echoed Mr. Kang’s reasoning. Others say the sign was confusing and the couple should not be blamed.