The Option Set custom control allows you to change the default behavior of a 2 values Option Set or 3 values Option set into a 2 or 3 tiles displayed side by side.
For the purpose of demonstrating this issue, I created four local option set controls. Two options sets will contain 2 values and two option sets will contain 3 values. The table below shows you the settings of these option sets that I created:
||Preferred Payment Method
||Email Delivery Status
|# of Option Sets
When I open the form, I will see the image below. Two of the option sets have no selected value, and two of them have a selected value.
I can go ahead, and select the different options in all of the controls, but if you notice the difference in behavior, the custom controls that had a default value, must have a value. I am unable to select that default value and leave it as null.
I would consider this to be a bug. If the setting of the control was required, I can understand that one of the values must be there in order to save, and I cannot change this from having at least one selected value, however, having a default value should allow me to remove that value and not select any other value.
I will report this to Microsoft and update this in case I hear any response.