Hello Phiten,
Yes, this is expected behavior.
Whether the box is validated or not is not in the control of the plugin.
Chrome is no different, it also renders the CSS for failed validation from the start. But in Chrome the default styling for failed validation does not include a red gradient border, so you don’t see it.
Again, this is normal. If you wish to change how inputs that fail validation are rendered, see the :invalid pseudo-selector. You should be able to use CSS in your child theme to change or hide it.
Hope this helps.
kind regards