EDIT - [Oops Linked the game not the editor!]
For some reason , even though an identical loop is used to create the main layout ( Press space to fill with red blocks, backspace to clear) , when it does the same for the thumbnail image, if fails to draw the last line.
I'm doing nothing different in the smaller loop, other than changing the positional offset/scale on the output, the reading via two loops is still the same.
You can see in the example image I've drawn a green border on the perimeter which is written to the array, however it fails when output again to the thumbnail size ( as you can see the bottom green line is misisng)
Download the CAPX
The capx uses Mipey's Sprite Font plugin - which I'd recommend everyone has. :)