I’m afraid I couldn’t reproduce this issue on my end. It’s possible that there’s a function on your site that overrides ACF’s validation. Could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Sixteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
If the issue persists, could you please share the JSON export of your field group so I can test it out on my installation?
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