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;
}