The same model converged in keras, but not in tensor flow, how is this possible?

I'm trying to work with lstm in tensorflow but got to the point where I can't make a simple imdb mood model to merge.

I took the keras model and tried to repeat the same model in tensorflow, in keras it trains and converges, however in tensorflow it just gets stuck at some point (0.69 loss).

I tried to make them as the same as possible, the only difference I can tell is that in keratas the padding is in front of the sequence, while in tensorflow I use post-padding due to conventions in tensorflow.

Any ideas what happened to my tensorflow model?

from __future__ import print_function

import random
import numpy as np

from tensorflow.contrib.keras.python.keras.preprocessing import sequence
from tensorflow.contrib.keras.python.keras.models import Sequential
from tensorflow.contrib.keras.python.keras.layers import Dense, Dropout, Activation
from tensorflow.contrib.keras.python.keras.layers import Embedding
from tensorflow.contrib.keras.python.keras.layers import LSTM
from tensorflow.contrib.keras.python.keras.layers import Conv1D, MaxPooling1D
from tensorflow.contrib.keras.python.keras.datasets import imdb

import tensorflow as tf

# Embedding
max_features = 30000
maxlen = 2494
embedding_size = 128

# Convolution
kernel_size = 5
filters = 64
pool_size = 4

# LSTM
lstm_output_size = 70

# Training
batch_size = 30
epochs = 2


class TrainData:
    def __init__(self, batch_sz=batch_size):
        (x_train, y_train), (_, _) = imdb.load_data(num_words=max_features)


        y_train = [[int(x == 1), int(x != 1)] for x in y_train]
        self._batch_size = batch_sz

        self._train_data = sequence.pad_sequences(x_train, padding='pre')

        self._train_labels = y_train

    def next_batch(self):
        if len(self._train_data) < self._batch_size:
            self.__init__()

        batch_x, batch_y = self._train_data[:self._batch_size], self._train_labels[:self._batch_size]
        self._train_data = self._train_data[self._batch_size:]
        self._train_labels = self._train_labels[self._batch_size:]

        return batch_x, batch_y

    def batch_generator(self):
        while True:
            if len(self._train_data) < self._batch_size:
                self.__init__()

            batch_x, batch_y = self._train_data[:self._batch_size], self._train_labels[:self._batch_size]
            self._train_data = self._train_data[self._batch_size:]
            self._train_labels = self._train_labels[self._batch_size:]

            yield batch_x, batch_y

    def get_num_batches(self):
        return int(len(self._train_data) / self._batch_size)

def length(sequence):
    used = tf.sign(tf.abs(sequence))
    length = tf.reduce_sum(used, reduction_indices=1)
    length = tf.cast(length, tf.int32)
    return length


def get_model(x, y):
    embedding = tf.get_variable("embedding", [max_features, embedding_size], dtype=tf.float32)
    embedded_x = tf.nn.embedding_lookup(embedding, x)
    print(x)
    print(embedded_x)
    print(length(x))

    cell_1 = tf.contrib.rnn.BasicLSTMCell(lstm_output_size)
    output_1, state_1 = tf.nn.dynamic_rnn(cell_1, embedded_x, dtype=tf.float32, scope="rnn_layer1",
                                          sequence_length=length(x))

    # Select last output.
    last_index = tf.shape(output_1)[1] - 1
    # reshaping to [seq_length, batch_size, num_units]
    output = tf.transpose(output_1, [1, 0, 2])

    last = tf.gather(output, last_index)

    # Softmax layer
    with tf.name_scope('fc_layer'):
        weight = tf.get_variable(name="weights", shape=[lstm_output_size, 2])
        bias = tf.get_variable(shape=[2], name="bias")

    logits = tf.matmul(last, weight) + bias

    loss = tf.losses.softmax_cross_entropy(y, logits=logits)

    optimizer = tf.train.AdamOptimizer()
    optimize_step = optimizer.minimize(loss=loss)

    return loss, optimize_step


def tf_model():
    x_holder = tf.placeholder(tf.int32, shape=[None, maxlen])
    y_holder = tf.placeholder(tf.int32, shape=[None, 2])
    loss, opt_step = get_model(x_holder, y_holder)

    with tf.Session() as sess:
        sess.run(tf.global_variables_initializer())

        step = 0
        for epoch in range(10):
            cost_epochs = []
            train_data = TrainData()
            cost_batch = 0
            for batch in range(train_data.get_num_batches()):
                x_train, y_train = train_data.next_batch()
                _, cost_batch = sess.run([opt_step, loss],
                                         feed_dict={x_holder: x_train,
                                                    y_holder: y_train})

                cost_epochs.append(cost_batch)


                step += 1
                # if step % 100 == 0:
                print("Epoch: " + str(epoch))
                print("\tcost: " + str(np.mean(cost_epochs)))



def keras_model():
    # print('Loading data...')
    (x_train, y_train), (x_test, y_test) = imdb.load_data(num_words=max_features)

    y_test = [[int(x == 1), int(x != 1)] for x in y_test]

    x_test = sequence.pad_sequences(x_test, maxlen=maxlen, padding='pre')

    model = Sequential()
    model.add(Embedding(max_features, embedding_size, input_length=maxlen))

    model.add(LSTM(lstm_output_size))
    model.add(Dense(2))
    model.add(Activation('softmax'))

    model.compile(loss='categorical_crossentropy',
                  optimizer='adam',
                  metrics=['accuracy'])

    print('Train...')
    data = TrainData()
    model.fit_generator(data.batch_generator(), steps_per_epoch=data.get_num_batches(),
                        epochs=epochs,
                        validation_data=(x_test, y_test))


if __name__ == '__main__':
    # keras_model()
    tf_model()

      

        
        
        
      

    

EDIT

When I limit the length of the sequence to 100, both models converge, so I assume there is something different in the lstm layer.