Question: During my game today, a player called a discarded tile at the same time another player picked their tile from the wall and racked it. What happens when this occurs?

Answer: In the event a player calls a tile, whether for exposure or for Mah Jongg, at the same time a player picks a tile from the wall, the player who called the tile gets preference and the player who picked from the wall must put the tile back exactly from where it came.  

Source: Mah Jongg Made Easy 2024, page 18 #10 (d)