I don’t know if this is a bug or not. But personnally I find it weird.
If I use a Combobox it is possible to edit the field directly without the combobox. The only way (i know) to prevent this is making the field uneditable. Now the user can only the values of the combobox.
But shouldn’t a field that is uneditable…be uneditable i.e. you cannot make changes at all??