There's no particular reason why it should be even at all. Just as the undersides of clouds aren't necessarily evenly lit. It depends on the perspective of the viewer and the position and shape of the cloud. The cloud (contrail) here isn't in a direct line of sight to the sun, but off to one side. Could be that the plane was descending as well, adding another angle to the mix. This would also explain why it just stops, too. The plane likely passed down to some altitude where the conditions were no longer right to produce a contrail.
Compared to clouds jet contrails are very thin, very even and very small. Of the thousands of contrails I have seen I have never seen one that was unevenly lit from edge to edge. Along the length is another matter. Which you will notice is very evenly lit apart from the ‘bottom most’ point.