multi-checkbox not working as expected

multi-checkbox behaves oddly when set as a required field.

I created a very simple multi-checkbox asking users whether they wish to receive our e-newsletter. There were two options, yes and no. I set it as a required field and in the advanced tab set a minimum of 1 selected value and a maximum of 1 selected value. However, the validation will only let me submit the form if I check both values.

Is this a bug, or have I misunderstood how multi-checkbox is supposed to work?

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