Hi,
My guess is that the model has overfitted and outputs the same predictions after epoch 1.
If this is the case, the question is why and how to get around it. Perhaps your dataset contains 90% of one of the classes which will bias the model to predict only that class in the evaluation.
You can verify this by printing/saving some out the model outputs after each epoch. If I were you I’d try to save the predictions as images, each class gets its own color. That should help with understanding what the model does.