Is this a new behavior? When creating a new thread, it used to be that XenForo would show threads with similar words in the title. But now, that list is collapsed:
…instead of…
This “feature” has led to there being two identical titles in the TV Shows subforum.
There are no settings for this, but there have been a LOT of updates in the background of may plugins getting us ready for Xenforo 2.3 which is now in beta testing. So one of those updates may have did this.
The fix I have found is on the far right there is a tiny blue arrow which expands the suggestions box, and it seems to remember that choice.