Superform stack number entry area

Hi, I have seen a bit of an odd problem this week with a site I have built. I created a site using a contact form stack, Superform, from onelittledesigner.
I have found that for some reason, when a website user trys to enter their telephone number, if they use the numberpad area of the keyboard, it will not work. If they use the numbers across the top of the keyboard, it will work.
Has anyone see this happen before, and if so what is the cause and what is the fix?
The page in question is here:

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