I’m afraid that I couldn’t reproduce this issue on my end. It’s possible that your site has a function that modifies the returned values. To make sure, could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Fifteen) 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.
Also, could please make sure that you have the latest version of ACF and WordPress?
If the issue persists, could you please open a new ticket here: https://support.advancedcustomfields.com/new-ticket/, explain the issue again and provide temporary admin credentials in the ticket?
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