I'm not sure if this helps, but I tried changing a line of code (24) in the TextureBackground.gd file. This makes sure that margin property is always set, instead of testing it against the max_width. After more testing I will post if the change worked.
So:
if self.rect_size.x > max_width:
$Text.autowrap = true
self.margin_right = max_width / 2
self.margin_left = -max_width / 2
Became:
if true:
$Text.autowrap = true
self.margin_right = max_width / 2
self.margin_left = -max_width / 2