The ability to use adaptive forms without the forms add on package is for demo purposes only. To use adaptive forms in your production environment, you have to license AEM Forms
The best option is to open it using Adobe PDF viewer. Previously browsers would allow Adobe PDF viewer to be the default PDF viewer, hence you may not have had this problem
I just tried the form you provided in the link using Acrobat and it works fineis it happening when you open the form using the pdf viewer of the browser?