I am unable to replicate this issue on my end. Are you running the latest version of ACF v 5.4.8?
Can you reproduce this issue when all other plugins are deactivated and while running one of the stock WP themes? If so, please open a new support ticket at email@example.com with more info on your configuration in form fo screenshots so that I can help you further.
Do you have any luck with this issue yet? I believe the issue is related to a JS conflict with your theme or other plugins. 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.