I just installed the Quform 2.1.0 and testing the Calendar Date picker. When a date was selected from the date picker, the date field could be updated.
If I tried to reselect another day in the same month again, the pop-up date picker has no response that it would not update the date field. However, if I try to switch to another month first by using the depth view, the selected date could be updated.
P.S. Quform 2.0.1 has no issue to re-pick the date in the same month.