Model's Accuracy Drops A Lot after converting

[MT010104]

Describe the bug
I trained a LeNet5 model with MNIST under Pytorch and used pytorch2keras to transfer it to a Keras model. But I found that the accuracy of the Keras model is only about 10% while the Pytorch model's accuracy is close to 100%. VGG16 models trained with CIFAR10 didn't show a similar situation. For consistency, no additional processing is performed on the data in either framework. I‘ve verified that it's not the data or the model itself that's wrong.

To Reproduce
Snippet of your code

class LeNet(nn.Module):
def __init__(self):
        super(LeNet, self).__init__()
        self.conv1 = nn.Conv2d(1, 6, 5)
        self.conv2 = nn.Conv2d(6, 16, 5)
        self.fc1 = nn.Linear(256, 120)
        self.fc2 = nn.Linear(120, 84)
        self.fc3 = nn.Linear(84, 10)
    def forward(self, x):
        out = F.relu(self.conv1(x))
        out = F.max_pool2d(out, 2)
        out = F.relu(self.conv2(out))
        out = F.max_pool2d(out, 2)
        out = out.view(int(out.size(0)), -1)
        out = F.relu(self.fc1(out))
        out = F.relu(self.fc2(out))
        out = self.fc3(out)
        return out

(x_train, y_train), (x_test, y_test) = mnist.load_data()
x_test = x_test.astype('float32') / 255.0
x_test = x_test.reshape(dataset_name_dict[name])
y_test = keras.utils.to_categorical(y_test, 10)
model = keras.models.load_model("models/mnist_lenet5.h5")
model.summary()
model.compile(loss='categorical_crossentropy', optimizer=keras.optimizers.SGD(lr=1e-3, momentum=0.9), metrics=['accuracy'])
y_predict = np.argmax(model.predict(x_test, verbose=0), axis=1)
origin_acc = model.evaluate(x_test, y_test, verbose=0)[1]
print(f"origin_acc : {origin_acc}")

Expected behavior
I would like to know why the accuracy drops and if there is a solution.