As you said you have to pass the column names as strings in set_format: columns=['hr', 'lr'].
However this is the feature type of the data itself of each column that may actually cause the issue.
When I was talking about string type, I was referring to the type of the data inside each column (for example the type of augmented_dataset[0]["hr"])
You can check if the feature type of each column with
print(augmented_dataset.features)

or you can directly check the first row augmented_dataset[0]