My two cents:
Based on what I've read, I would doubt that the sunlight itself is causing any problems. We're talking a February sun only a couple hours a day (since east facing window, not south) and filtered through a window and on a cloudy day.
Humidity could be an issue in a heated location. It is probably only 30 percent relative humidity, so the cutting and leaves will lose moisture to the air, particularly if a green cutting or new leaves. Even if the roots look good, there might not be enough vascular action to replenish the moisture losses.
Did you say that you are only having this problem on 3 out of 100 cuttings? If so, I'd say you have nothing to worry about, and that your success rate is great. Try to make as many observations as you can about the specifics of these vs the others. For example, I have about 50 cuttings that are doing great, but one of them dried up shortly after I put it in into a 1 gallon pot. One difference with this cutting is that the roots did not form from the bottom like most of the others, but a ton of roots came out near the top of the cutting. I did my best to cover these roots with soil when I planted, but now I realize that they were just barely covered. So yesterday I looked at them and they're all brown and dried out. The top half inch of soil dried out, and that is where the roots were connected to the cutting. So now I have a dry twig. An opportunity to learn and make sure I don't make that same mistake next time.
Rob