update("$push") does not work with Persistable

I have document structure similar to that:
`class A {
    /** @var B[] */
    private $_b = [];
}`
Both A and B are persistable. So, when I insert A with multiple Bs via driver, it's OK. I see __pclass field in mongo shell.
When I execute something like (of course, again via php api)
`update({"$push": {"b": {new B object}}})`
no __pclass field is created and unserialization goes wrong.
