I had a personal list called “Batman Collection” which included Batman movies from different franchises. Then at some point it removed all the items from other franchises besides the official TMDB collection (Batman Collection — The Movie Database (TMDB)).
This started happening a while ago but I just figured out the cause. And I also changed the name of my personal list from “Batman Collection” to just “Batman”, and now all the other items in the list are showing up ( Trakt Web: Track Your Shows & Movies ).
The same thing happened with John Wick, except I created a new collection called “John Wick Collection” and it automatically contained the 4 primary John Wick movies, and other items I added weren’t showing up in the list until I renamed it to just “John Wick”