In the new "List" tab of a project, users can create a custom filter and save it - this will create a filter that is shared with all users of this project. This creates two issues:
1. Users are probably unaware that they created a shared filter and expect it to be private by default.
2. Admins cannot prevent this functionality for users with read-only permissions on the project.
So my product suggestion would be to either make those filters private by default, or add a project-level permission to disable this functionality for a given role.