This TV does not have an S-video input.
On other TVs that do have both inputs, I've never seen that problem using either one.
Do you mean that you've tried the Roku box on other TVs with a composite input and haven't seen the problem?
Actually, lemme ask this... Do you have another TV in the house with a composite input? If so, does the Roku box work there?
Do you have anything else in the house with a composite output that you can try with this TV?
Do you have a neighbor/friend with either side of the equation that you can use for a test?
I think the odds of you being 2/2 for getting a box that has a wonky composite output are slim, I'd start looking at other things. The way the video chip is in this box, I don't think the composite signal has the possibility of losing JUST the color burst info.