It was possible to click on an option to unselect it. That is no longer possible. Reloading the page does not reset the selected options either. So basically if you make a mistake there is no way to undo it.
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In my experience, what you have described is standard behaviour for those type of items and I've never seen Forms have the ability to unselect an answer from Choice or drop down list once selected.
I would suggest if it's a big issue to add an extra option in where people can select N/A or something like that if something was accidently chosen.
Best wishes with your Forms journey!