neural network to find a very simple linear model (scikit-learn)
The problem is relu which is the default activation function. It zeros the inputs smaller than 0. Problem will be solved by using other functions. For example:

C = NN.MLPRegressor(activation="identity", hidden_layer_sizes=(1,1),max_iter=25,random_state=1)


Note that even 1 hidden layer with 1 neuron is enough to find the $X_2 = X_1$ decision boundary.