When the items control is rendered, it checks to see if the current item is of its item type, in case of ComboBox - ComboBoxItem, if so it does not create a new comboboxitem and just adds the item you gave it. And one item cannot have two parents, hence
the wierd behavior.
In case of setting items source as collection of ints, since each item is an int and not comboboxitem, it creates a new comboboxitem and gives it the int. Hence works fine.
again no expert, but i think thats what the behavior is.