The refresh_unsupported_items interval is not fully used in case of active checks. There is ZBXNEXT-2633 feature request already.
Currently when the agent receives unsupported item in the active checks list then it attempts to refresh that item during next previously scheduled check. Server will not include unsupported item into the list until refresh unsupported items interval elapsed. That helps to prevent too frequent checks if refresh_unsupported_items interval is more than item update_interval.
But in case refresh_unsupported_items interval is less than item update_interval the attempt to refresh the item still will be made during next previously scheduled check. Which might be much later than refresh_unsupported_items interval elapsed since last item check.