Comment 2 for bug 1773945

My guess is as follows:

In nova,
if the record of the marker VM instance exists in the cell
but the 'cell_mapping' in the InstanceMapping of the marker VM instance is null (None),
the issue occurs.

https://github.com/openstack/nova/blob/f902e0d5d87fb05207e4a7aca73d185775d43df2/nova/compute/instance_list.py#L56-L73

--------------------------------------------------------------------------
    def get_marker_record(self, ctx, marker):
        try:
            im = objects.InstanceMapping.get_by_instance_uuid(ctx, marker)
        except exception.InstanceMappingNotFound:
            raise exception.MarkerNotFound(marker=marker)

        elevated = ctx.elevated(read_deleted='yes')
        with context.target_cell(elevated, im.cell_mapping) as cctx:
            try:
                # NOTE(danms): We query this with no columns_to_join()
                # as we're just getting values for the sort keys from
                # it and none of the valid sort keys are on joined
                # columns.
                db_inst = db.instance_get_by_uuid(cctx, marker,
                                                  columns_to_join=[])
            except exception.InstanceNotFound:
                raise exception.MarkerNotFound(marker=marker) <------ Here
    return db_inst
--------------------------------------------------------------------------

In nova-conductor, the record of the VM instance exists in the cell
but the 'cell_mapping' in the InstanceMapping of the VM instance is null (None)
between *1 and *2.

*1: https://github.com/openstack/nova/blob/f902e0d5d87fb05207e4a7aca73d185775d43df2/nova/conductor/manager.py#L1172-L1175
*2: https://github.com/openstack/nova/blob/f902e0d5d87fb05207e4a7aca73d185775d43df2/nova/conductor/manager.py#L1236-L1239