I'm still testing this, but I believe that this is showing people threads from other sections of a forum where they may not have permission to view.
Example:
Site member has access to forums: A, B & C
Site member does not have access to forum D.
Site member is reading a thread from forum B but the Similar Threads Plus results are showing threads from 'Forum D' where they shouldn't be able to see the content. (It shows title and name of forum where they shouldn't be able to see due to no access by user group permissions)
Can anyone reproduce this? Got a number of reports saying people were seeing things that they believe they shouldn't. Permissions were intact, they couldn't actually click on and view the thread, but the title and forum name was enough to cause some concern.
Edit: 2/7/2018 - Since there's been no update, we've pulled this from our site until design and actual intended function can be validated.