Search for plastic strip behind figure on rebelscum and you will find multiple threads talking about this. I will attach one such example.
It is fairly common but there seems to be no rhyme or reason behind which figures got the treatment. It is assumed this was done to protect the painted figure from the ink in the cardback but as far as I know nothing has ever been definitively said as to why it was done, and no reason on why some figures got it and some didnt.
I have caviar dreams and McDonald's pockets