ComponentBehavior: Bound causes 'QQmlContext: Cannot set property on internal context'

pragma ComponentBehavior:Bound causes delegate component of MonthGrid to warn about 'QQmlContext: Cannot set property on internal context'. A trivial delegate: Rectangle{} or delegate: Text{} (or probably any custom delegate I think) can cause the warning.

However, this warning message is not observed for similar model/view items such as ListView or GridView. A MonthGird with no custom delegate component does not show the problem either. So far, only MothGrid with custom delegate exhibits such behavior.