Issues with time slot on iPhone

Hi,
I’m having issues with iPhones being able to select a time from the ‘select time slot’ field. On the iphone, the menu either disappears quickly when clicked on or just doesn’t display the time at all. I’ve seen someone has said that this is to do with admin-ajax, but what do I need to amend in admin-ajax to resolve it please?
Brilliant-plug in.

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