My test was on a site that is the latest version of WP with the latest default theme and only ACF active. Everything is working as expected. I don’t know what you mean by
*WordPress v4.9.1 (Japanese version -> reinstall US version)
If the validation is not working then you have something that is interfering with the operation of the JavaScript and AJAX request associated with it, or for some reason ACF is detecting that the field is not empty. I would try deleting the field and creating a new one and trying it again. Beyond that I can’t give you much advice. You could try submitting a new support ticket and see if they can help you https://support.advancedcustomfields.com/new-ticket/
