Actually, if you look at the photo, you can se the reflection is the brightest at the bottom of the contrail. You aren’t see it fall into shadow at all, the darkness across the full width of the bottom is low level haze in the foreground.
I guess you see what you want to see. The bottom of the contrail is clearly in front of those clouds and in shadow. It also passes in front of both lines of clouds. The ones on the horizon and the one higher in the pic and therefore much closer.