Running on an application with SF2.8, before migrating it to SF3. What I got here is, at some point in a form

    // BarType.php
    $builder->add('foo', CollectionType::class, [
       'entry_type'     => new FooType(),
       'error_bubbling' => false,
       'allow_add'      => true,
       'allow_delete'   => true,
    ]);

Reading this I found that we need to change entry_type of forms from new FooType() to FooType::class.

But I've noticed something very weird: if I change entry_type from instance to FQCN and in embedded form (FooType) I use something like

    // FooType.php
    $resolver->setDefaults([
        'data_class' => 'Vendor\Bundle\Entity\Foo',
        'empty_data' => new Foo(),
    ]);

Then, every object added to the collection are "collapsed" into the same one (e.g.: I add three elements, after form binding I got three Foo elements in my Bar object but they're all the same).

Unfortunately, now, I cannot write an example to let give you a better understand of what is going on here but I will do in the next days if anyone need it.