This issue is tested, one issue is the encoding.
fl.edit_features(
adds=[
{
'attributes': {
'SpecialCharacters': b'ABC\xc3\xa4\xc3\xbc\xc3\xb6\xc3\xa8\xc3\xa0\xc3\xa9\xc3\xa8\xc3\xab\xc3\x9f'.decode()
}
}
]
)
The data is encoded then decoded back properly. The result shows properly.