The value of the TextBoxField is not visible until the field is clicked.

Workaround: Force content update:

foreach (var widget in textBoxField.Widgets)
{
    widget.RecalculateContent();
}

If the update still doesn't fix the issue, change the font prior to setting the value of the field:

foreach (var widget in textBoxField.Widgets)
{
    widget.TextProperties.Font = FontsRepository.Helvetica;
}