iOS users signature

Hi. Here is the problem. I have Acrobat Pro. I have prepared a form with required signature. When I send that form to the customers with iPhones, they fill in the form and send it back to me, and the form is still editable. The costomers say that the form does not request signature. However, if I send the form to Windows users, they fill it in, put signatures, and the form becomes ineditable. How to fix the issue on iPhones? Please advise.