QFileDialog Sheet Improper Behaviour

The easiest way to describe what I'm talking about is through the attached gif.

I have setup a QFileDialog in my code as follows:

QFileDialog* dialog = new QFileDialog(this, Qt::Sheet);

dialog->setWindowModality(Qt::WindowModal);

if(dialog->exec() == QFileDialog::Accept){...}

The intent is to get a sheet as many applications in macOS, this code gets me close until right after the animation where it snaps out as if it were just set to modal. 

On another project I work on, I use dialog->open() and go through the effort of setting up a slot to handle the file saving. The sheets behave perfectly like this and I have none of the snapping that occurs after. However, this can be a pain to have split off into a separate handler to handle this.. Is there something wrong in my code or is this a bug in Qt? This bug has happened in Qt 5.8.0 and 5.9.0 for me on both OS X 10.11 and macOS 10.13 Beta.

Any further information needed I will be happy to provide, cheers!