I think I might know why. The 'return' key might be triggering a redirection (form send) event for some reason.
This should be solveable with a custom code. Add this custom code to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case!
Please use and FTP client to do this change. In case you need help, just leave temporary FTP details and I will do it for you.Best,
If you like my products, don't forget to rate them on codecanyon :)