Django forms.ModelMultipleChoiceField appearance is different when an instance is passed to the form and when the form is empty

i have this ModelMultipleChoiceField,

    services = forms.ModelMultipleChoiceField(
        queryset=Services.objects.all(),
        widget=forms.SelectMultiple(attrs={'class': 'select2'}),
        required=False
    )

when the i display an empty form it appears like that: without instance

but when i pass an instance to the form the field appears like that: with instance

i want both to have the look of the first screenshot

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