This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
Sometimes it's not catching certain polys as vertex paint targets. Looks like it happens when there's more than one layer of forward facing geometry over the highlighted area, and it just takes the first poly it finds; needs to find all candidates and use the closest one to the camera.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-37076 in the post.