When string is broken with pistons, the color of it is still the dark-gray color, not the white one it should be. I tested it some more and found that if you place back the string in a certain order, the newly placed ones would stay white and the old ones would stay dark-gray, but when placed the other way, it would fix itself. If the pistons that broke the string were facing south or east, you would have to place the string on the right first to keep the glitch. If the pistons were facing north or west, you would have to place the first string on the left. Even after updating the string, it would not change. The only way I found to change it was to break the string or the tripwire hooks. From what i saw, it doesn't affect the function of it at all, it seems to be just visual.