Vertical line of dead pixels on 50g LCD

[BINUBALL]

I found this thread while googling. When I use newRPL, If there are many numbers and string on stack like below. There are vertical light black line around numbers and string.

I feel uncomfortable with this. Is this dead pixels? I've tested 50g's screen pixels by making 131×80 black grob. But I think every pixel is ok on this test.

Code:

::
80 131 MAKEGROB INVGROB
;
@