Editor Select2 plug-in, Bootstrap theme breaks the select field

I am using Editor-1.7.3 with https://editor.datatables.net/plug-ins/field-type/editor.select2 plug-in and Select2 Bootstrap theme https://github.com/select2/select2-bootstrap-theme.
The demo can be viewed at https://nfctech.eu/datatables/editor-select2-bootstrap.html.
When I click on the "New" button and the form modal opens, the use of Select2 Bootstrap theme breaks the select field.
The code used in the demo is

{
            "label": "NAZIONE*:",
            "name": "nazione",
            "type": "select2",
            "options": [
              "Italia",
              "Francia",
              "Germania",
              "Russia",
              "Stati Uniti"
            ],
            "opts": {
            "placeholder": "Seleziona una nazione",
            "allowClear": true,
            "theme": "bootstrap"
            }
          },
          {
            "label": "AREA*:",
            "name": "area",
            "type": "select2",
            "options": [
              "Francia",
              "Nord Italia",
              "Russia",
              "Americhe"
            ],
            "opts": {
            "placeholder": "Seleziona un'area di competenza",
            "allowClear": true,
            }
          },

where the select "NAZIONE" makes use of the Select2 Bootstrap theme and the select "AREA" no.
As you can see from the demo,:
- the select "NAZIONE" has no border at all and the Bootstrap theme is applied only partially when the select is opened
- the select "AREA", that does not use the Select2 Bootstrap theme, is displayed correctly according to the default Select2 theme.
I checked the implementation and it seems correct. Any idea about the cause of this issue?
Thanks