I don’t really have a complete answer, but here are some ideas.
parsed_features = {‘raw_data’: raw_data_sp_tensor, }
That’s just creating a dict, so I think the error is really with:
transformed_features = self.model.tft_layer(parsed_features)
Does that line work in serve_tf_examples_fn? If so, then that’s a clue to the problem.  Fundamentally though you’re giving it a sparse tensor and it wants a string.