I use saturation -100 to set the google map to grayscale. Everything works fine until recently the interstate label show in the original colors (blue and red). Is this a google map glitch or is there a way to make the interstate label change back to grayscale?

I don't think there is any problem with my code because it has the same problem on the Styled Map Wizard that google provide. To test, just change the saturation to -100 and zoom in till you can see the interstate label.