That does appear to be a bug.  Note that the new email API handles it correctly:

    >>> x = """
    ... > From: =?utf-8?Q?zq@redacted.com.cn=E3=82=86=E2=86=91=E3=82=86?=
    ...  =?utf-8?Q?=E3=82=83=E3=82=85=E3=81=87=E3=81=BA=E3=81=BD=E3=81=BC"\=E3?=
    ...  =?utf-8?Q?=81=A9=E3=81=A5=E3=81=A2l=E3=81=A0=E3=81=B0=E3=81=A8=E3=81?=
    ...  =?utf-8?Q?=8FKL=E3=81=84=E3=82=8C=E3=82=8B=E3=82=86>KL=E3=82=89JF?=
    ...  <mxvu@redacted2.com>
    ... """
    >>> from email import message_from_string
    >>> from email.policy import default
    >>> m = message_from_string(x+'\n\ntest', policy=default)
    >>> m['from']
    '"zq@redacted.com.cnゆ↑ゆ ゃゅぇぺぽぼ\\"\\\\� ��づぢlだばと� �KLいれるゆ>KLらJF" <mxvu@redacted2.com>'
    >>> m['from'].addresses[0].addr_spec
    'mxvu@redacted2.com'
    >>> m['from'].addresses[0].display_name
    'zq@redacted.com.cnゆ↑ゆ ゃゅぇぺぽぼ"\\\udce3 \udc81\udca9づぢlだばと\udce3\udc81 \udc8fKLいれるゆ>KLらJF'

I'm not particularly interested myself in fixing parseaddr to handle this case correctly, since it is the legacy API, but if someone else wants to I'll review the patch.