Please ensure that you have opened the PDF form using Adobe Acrobat Reader DC for mobile. The iOS PDF viewer can't properly handle PDF forms which is why you might be facing this issue.
You can refer the following links:
Hope that helps.
What is the version of your Adobe Reader? What is the iOS version on your iPad?
Would you share your PDF form with us (Adobe) so that we can investigate the cause of the problem? I have sent you a separate email with the instructions on how to share your document.
The FAQ documents that you listed in your reply above are marked as outdated.
The latest version (15.x) of Acrobat Reader for iOS runs only on iOS 8.
Please refer to the following FAQ documents for Acrobat Reader for iOS version 15.0 or newer.
The links to the FAQ documents are also available in the Frequently Asked Questions section of the CLOSED Reader/Acrobat DC for mobile (Read Only) forum page.