Sorry for the issue, this little CSS bug has caused a few headaches!
I have pushed a hotfix for this issue.
Can you please re-download the ACF PRO plugin files and test the issue.
An easy way to re-download is to edit the main acf.php plugin file and change the version number ‘5.6.6’ down to ‘5.6.5’
– please note the version number appears twice in this file and both will need to be changed
– after this change is made, please update the plugin as per normal and it will update to 5.6.6 (including the new fix)
Please be sure to hard refresh your browser to clear cached CSS rules.