While trying out few things in July 2017 update of Dynamics 365, we encountered a strange issue.
The members count field was properly in case of Static Marketing List, however, while converting the Dynamic Marketing List to Static (using Copy To Static), this field was not getting populated.
And also, the field was getting unlocked in case of Update Form for both Static and Dynamic Marketing list, unlike earlier version where this field was always locked on Form.
The field properly calculating count and locked in case of earlier versions.
Hope it helps..