delta lake partitioning generates wrong values for folder names

Question

I am trying to partitioning the dataframe using a column GROUP. I have code like this

df = DataFrame(
    data={
        'NAME': ['John', 'Max', 'Henry'],
        'GROUP': ['A', 'B', 'A']
    }, 
    schema={
        'NAME': pl.Utf8(),
        'GROUP': pl.Utf8()
    }
)

df.write_delta(
    'current_folder/',
    delta_write_options={
        'partition_by': ['GROUP']
    }
)

But upon execution, instead of this

current_folder/
    _delta_log/
    GROUP=A/
    GROUP=B/

I get this folder structure

What is the problem? When I use other polars type like pl.Int64() for partitioning, it works properly.