This is expected (but annoying) behaviour as per: https://code.djangoproject.com/ticket/1130

Two options are possible:

1. Rewrite get_field_value to dynamically determine if the field is capable of returning multiple values
2. Create a new get_field_values to call getlist() instead of get()

Proposed rewrite:

def get_field_value(form, field_name):
    """
    Return the current bound or initial value associated with a form field, prior to calling
    clean() for the form.
    """
    field = form.fields[field_name]
    
    if form.is_bound:
        # Get list or get single value
        if isinstance(field, MultipleChoiceField):
            data = form.data.getlist(field_name)
        else:
            data = form.data.get(field_name)
        
        if hasattr(field, 'valid_value') and field.valid_value(data):
            return data

    return form.get_initial_for_field(field, field_name)

@DanSheps to submit a PR per maintainers meeting

@DanSheps could you please rewrite the original issue to include reproduction steps? I'm not able to reproduce this:

>>> from django.http import QueryDict
>>> from utilities.forms import get_field_value
>>> from dcim.forms import InterfaceForm
>>> 
>>> qd = QueryDict("mode=tagged&tagged_vlans=2&tagged_vlans=27")
>>> f = InterfaceForm(initial=qd)
>>> get_field_value(f, 'tagged_vlans')
['2', '27']

After re-reviewing the code, I realised I made an error before trying to fix this:

On March 31st, when fixing another issue, the code around this function changed from:

    if form.is_bound and (data := form.data.get(field_name)):
        if hasattr(field, 'valid_value') and field.valid_value(data):
            return data

to

    if form.is_bound and field_name in form.data:
        if (value := form.data[field_name]) is None:
            return
        if hasattr(field, 'valid_value') and field.valid_value(value):
            return value

This slight change in code seems to have resolved the issue fully as we are no longer using .get() to retrieve the field value.

Fixed by: #19023