Can't set to None relationship property of object being in collection of collections

Test case:
from sqlalchemy import Column, Integer, String, ForeignKey, create_engine
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.ext.declarative import declarative_base


Base = declarative_base()


class DocLink(Base):
    __tablename__ = 'DocLink'
    id = Column(Integer, primary_key=True)
    block_id = Column(ForeignKey('DocLinkBlock.id', ondelete='CASCADE'),
                      nullable=False)
    block = relationship('DocLinkBlock',
                         primaryjoin='DocLink.block_id==DocLinkBlock.id')
    ref_url = Column(String(250), nullable=True)
    ref_doc_id = Column(ForeignKey('Doc.id'))
    ref_doc = relationship('Doc', primaryjoin='DocLink.ref_doc_id==Doc.id')


class DocLinkBlock(Base):
    __tablename__ = 'DocLinkBlock'
    id = Column(Integer, primary_key=True)
    doc_id = Column(ForeignKey('Doc.id', ondelete='CASCADE'), nullable=False)
    doc = relationship('Doc')
    links = relationship(DocLink, cascade='all, delete-orphan')


class Doc(Base):
    __tablename__ = 'Doc'
    id = Column(Integer, primary_key=True)
    link_blocks = relationship(DocLinkBlock, cascade='all, delete-orphan')


engine = create_engine('sqlite://', echo=True)
Base.metadata.create_all(engine)
session = sessionmaker(bind=engine)()

ref_doc = Doc(id=1)
session.add(ref_doc)
session.commit()

doc = Doc(
    id=2,
    link_blocks=[
        DocLinkBlock(
            id=1,
            links=[
                DocLink(
                    id=1,
                    ref_url='url',
                    ref_doc=ref_doc,
                ),
            ],
        ),
    ],
)
session.add(doc)
session.commit()
assert doc.link_blocks[0].links[0].ref_url=='url'
assert doc.link_blocks[0].links[0].ref_doc is ref_doc

doc.link_blocks = [
    DocLinkBlock(
        id=1,
        links=[
            DocLink(
                id=1,
                ref_url=None,
                ref_doc=None,
            ),
        ],
    ),
]
session.commit()
assert doc.link_blocks[0].links[0].ref_url is None
assert doc.link_blocks[0].links[0].ref_doc is None # Fails
There is no such problem when DocLinkBlock.links is updated directly (no nested collections).
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