Normal text input shows entered text as dots

I have an order form with all the usual inputs: <input type="text">, <textarea> and so on. I use HTTPS for my website.

On very rare occasions (about 1 in 100), clients complain that when they enter text into an input, they don't see the letters entered and instead all that is shown on screen are dots (like the ones that appear when you enter a password).

These people are average internet users; they can't even tell me which browser they are using. So it's a shot in the dark for me.

I was not able to reproduce the problem on a variety of systems and can't seem to find anything related on the internet.

Help please?

Form starts like this:

<form action="phpscript.php" name="my_form" method="post" onsubmit="return validate(event, this)" autocomplete="on" novalidate="novalidate">

Web pages are encoded in UTF8.