Hi!
I was able to re-create the issue on my test environment and I found the solution. No need to change anything in the code, it was the so called “filtered” content doing the problem.
If open up related posts pro settings and go to the Content Options panel and scroll down, there is an option Run the content filter?: http://i.imgur.com/bngHMlh.png
Turn that off and the problem will go away 🙂