Can shuffling my training data lead to bad model performance?

I am writing a neural network in C++ to approximate the xSin(x) function using a single hidden layer with 5 hidden neurons. The hidden neurons use tanh activation and the output layer uses Linear activation. I used 30 training examples for 10,000 epochs.

Until I shuffled my data, this is what I got: (RED: Predicted data, GREEN: Actual Data), also the MSE was near 0

But when I shuffle the indices of the training examples and verify that my shuffling does shuffle, I get terrible results:

,

and the Error vs Epoch as:

What could possibly go wrong? Can shuffling be responsible for this?

Here is the simple code for reference

//Shuffle Function 
void shuffle(int *array, size_t n)
{
    if (n > 1) //If no. of training examples > 1
    {
        size_t i;
        for (i = 0; i < n - 1; i++)
        {
            size_t j = i + rand() / (RAND_MAX / (n - i) + 1);
            int t = array[j];
            array[j] = array[i];
            array[i] = t;
        }
    }
}


int main(int argc, const char * argv[])
{
    //Some other actions

    ///FOR INDEX SHUFFLING
    int trainingSetOrder[numTrainingSets];
    for(int j=0; j<numTrainingSets; ++j)
        trainingSetOrder[j] = j;


    ///TRAINING
    //std::cout<<"start train\n";
    vector<double> performance, epo; ///STORE MSE, EPOCH
    for (int n=0; n < epoch; n++)
    {

        shuffle(trainingSetOrder,numTrainingSets);
         for (int i=0; i<numTrainingSets; i++)
        {
            int x = trainingSetOrder[i];
            //cout<<" "<<"("<<training_inputs[x][0]<<","<<training_outputs[x][0] <<")";

            /// Forward pass
            for (int j=0; j<numHiddenNodes; j++)
            {
                double activation=hiddenLayerBias[j];
                //std::cout<<"Training Set :"<<x<<"\n";
                 for (int k=0; k<numInputs; k++) {
                    activation+=training_inputs[x][k]*hiddenWeights[k][j];
                }
                hiddenLayer[j] = tanh(activation);
            }

            for (int j=0; j<numOutputs; j++) {
                double activation=outputLayerBias[j];
                for (int k=0; k<numHiddenNodes; k++)
                {
                    activation+=hiddenLayer[k]*outputWeights[k][j];
                }
                outputLayer[j] = lin(activation);
            }



           /// Backprop
           ///   For V
            double deltaOutput[numOutputs];
            for (int j=0; j<numOutputs; j++) {
                double errorOutput = (training_outputs[i][j]-outputLayer[j]);
                deltaOutput[j] = errorOutput*dlin(outputLayer[j]);
            }

            ///   For W
           //Some Code

            ///Updation
            ///   For V and b
            ///Some Code

            ///   For W and c
            for (int j=0; j<numHiddenNodes; j++) {
                //c
                hiddenLayerBias[j] += deltaHidden[j]*lr;
                //W
                for(int k=0; k<numInputs; k++) {
                  hiddenWeights[k][j]+=training_inputs[i][k]*deltaHidden[j]*lr;
                }
            }
        }
      }


    return 0;
}

I found 2 mistakes (silly!) in the training part:

1)

/// Backprop
           ///   For V
            double deltaOutput[numOutputs];
            for (int j=0; j<numOutputs; j++) {
                double errorOutput = (training_outputs[i][j]-outputLayer[j]);
                deltaOutput[j] = errorOutput*dlin(outputLayer[j]);
            }

should have been

/// Backprop
           ///   For V
            double deltaOutput[numOutputs];
            for (int j=0; j<numOutputs; j++) {
                double errorOutput = (training_outputs[x][j]-outputLayer[j]);
                deltaOutput[j] = errorOutput*dlin(outputLayer[j]);
            }

2)

///   For W and c
            for (int j=0; j<numHiddenNodes; j++) {
                //c
                hiddenLayerBias[j] += deltaHidden[j]*lr;
                //W
                for(int k=0; k<numInputs; k++) {
                  hiddenWeights[k][j]+=training_inputs[i][k]*deltaHidden[j]*lr;
                }
            }

should have been

///   For W and c
            for (int j=0; j<numHiddenNodes; j++) {
                //c
                hiddenLayerBias[j] += deltaHidden[j]*lr;
                //W
                for(int k=0; k<numInputs; k++) {
                  hiddenWeights[k][j]+=training_inputs[x][k]*deltaHidden[j]*lr;
                }
            }

Post which, I get and