Código aquí:https: //codesandbox.io/s/github/nieroda/js_er

In función endTurn

    console.log(`GameBoard Before`)
    console.log(gameBoardCopy)

    gameBoardCopy[currentRow][4] = { numColorMatch: 2, numExactMatch: 2 }

    console.log(`GameBoard After`)
    console.log(gameBoardCopy)

Ver salida de consola

Before:
5: Array(5)
0: "BlueViolet"
1: "BlueViolet"
2: "BlueViolet"
3: "BlueViolet"
4: {numColorMatch: 0, numExactMatch: 0}

After 

5: Array(5)
0: "BlueViolet"
1: "BlueViolet"
2: "BlueViolet"
3: "BlueViolet"
4: {numColorMatch: 0, numExactMatch: 0}

¿No puedo entender por qué no funciona?