forms.ChoiceField calls choices callback twice?

I'm running:

Python 3.7

Django 2.2

Debian 4.9

If I configure a forms ChoiceField with a callback function for choices, it gets called twice each time I render the form:

from unittest import TestCase
from django import forms

def callback():
    print("callback")
    return [('foo', 'bar')]

class MyForm(forms.Form):
    f = forms.ChoiceField(choices=callback)


class FormTest(TestCase):
    MyForm().as_p()

prints:

./manage.py test spi.test_f
callback
callback
System check identified no issues (0 silenced).

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK

Is this a known issue?