Strange behavior in current Keras setting (tensorflow-1.7.0 Keras-2.1.5) when using CPU
I recently had a similar trouble with porting working models to tensorflow 1.7. I found that the initialization was the culprit. Adding 
model.add(Dense(2048, activation='relu', kernel_initializer=initializers.glorot_normal())) instead of model.add(Dense(2048, activation='relu')) fixed the problem.