Textfield border can't be disabled

This:
$pdf->setFormDefaultProp(array('lineWidth'=>0, 'borderStyle'=>'none', 'fillColor'=>array()));
should disable the border around the Textfield, if I understood the documentation correctly.
But it seems to only make it transparent and still draws it with a lineWidth of 1.

A little background to what I'm trying to achieve:
I'm trying to create a PDF file with a form. The text fields need to be precisely positioned to fit the image in the background. Some of the text fields have default values (via the 'v' attribute).

The issue is that when I click the text field to edit the contents, the text moves a few pixels to the side (probably because of the border around the active input field). When the input loses focus, the text does not return to the original position.

I've tried to visualize the problem with the screenshots in the attachment
(note the text position relative to the line below)

My form default settings:
$pdf->setFormDefaultProp(array('lineWidth'=>1, 'borderStyle'=>'solid',
'fillColor'=>array(), 'strokeColor'=>array(255, 128, 128)));

This is the code for the field:
$pdf->TextField('test', 47, 6, [], ['v' => 'test(12)'], 8, 13);

What I have tried so far (each and combinations of them):

• setting the lineWidth to 0, but while the border is no longer visible, the text still moves.
• setting the borderStyle to 'none', same result.
• setting the strokeColor to [], same result.

What I would like to do:

• set a padding to the default value (or otherwise influence its positioning)
  or
• remove the border from the active form field (not just make it invisible)