I tried to use 'group_by' option with a call to a static method.

$form = $this->createFormBuilder()
            ->add('files', 'choice', [
                'choices' => [/* Some Choices */],
                'choices_as_values' => true,
                'group_by' => 'MyStatic::method'
            ])
            ->getForm();

Here is a dummy code sample of what I'm doing.
I saw in Symfony\Component\Form\ChoiceList\Factory\PropertyAccessDecorator::createView line 203, the test is if (is_string($groupBy)).
I think it should be if (is_string($groupBy) && !is_callable($groupBy)).

I have test with option choice_value, and other new options, but this bug may exists for those ones too.