This issue may be related or a duplicate of #11461.

> For example codecs.utf_16_le_decode(b'\x00\xd8\x00') should return ('', 0), but raises UnicodeDecodeError.

Only incremental decoder should return partial results. Other decoders are strict and (usually) stateless.

$ ./python 
>>> import codecs
>>> decoder = codecs.getdecoder('utf8')
>>> decoder('\u20ac'.encode('utf8'), 'strict')
('€', 3)
>>> decoder('\u20ac'.encode('utf8')[:2], 'strict')
UnicodeDecodeError: 'utf-8' codec can't decode bytes in position 0-1: unexpected end of data