entities.php

<?php

/**
 * @Entity
 */
class A
{
    /**
     * @GeneratedValue()
     * @Id @Column(type="integer")
     */
    public $id;

    /**
     * @OneToOne(targetEntity="B", inversedBy="a")
     * @JoinColumn(nullable=false)
     */
    public $b;
}

/**
 * @Entity
 */
class B
{
    /**
     * @GeneratedValue()
     * @Id @Column(type="integer")
     */
    public $id;

    /**
     * @OneToOne(targetEntity="A", mappedBy="b")
     * @JoinColumn(nullable=true)
     */
    public $a;
}

/**
 * @Entity
 */
class C
{
    /**
     * @GeneratedValue()
     * @Id @Column(type="integer")
     */
    public $id;

    /**
     * @ManyToOne(targetEntity="A")
     * @JoinColumn(nullable=false)
     */
    public $a;
}

test.php

<?php

class BugTest extends \Doctrine\Tests\OrmFunctionalTestCase {

    protected function setUp() {
        parent::setUp();
        $this->_schemaTool->createSchema(
            [
                $this->_em->getClassMetadata(A::class),
                $this->_em->getClassMetadata(B::class),
                $this->_em->getClassMetadata(C::class),
            ]
        );
    }

    public function testExplicitPersist()
    {
        $a = new A();
        $b = new B();
        $c = new C();

        $a->b = $b;
        $b->a = $a;
        $c->a = $a;

        $this->_em->persist($a);
        $this->_em->persist($c);
        $this->_em->persist($b);
        $this->_em->flush();
    }
}