Question: What happens when two players call the same tile and the one who was entitled to the call displays an invalid Mah Jongg?

Answer: The other player who called the tile for Mah Jongg is entitled to the win. If that player were not granted the win, it would discourage others from discarding a winning tile, placing players at an unfair disadvantage.

Source: Mah Jongg Made Easy 2020, page 18(g)