User has assigned a few employees to a project using Project Assignments, but these employees are able to enter time on other projects as well. How to restrict the projects an employee can access for logging time?
In Core, assigning employees to projects does not restrict the projects for which they can create time and expense entries, but rather it restricts the employees who can create time and expense entries on the projects. In other words, Project Assignments restricts which employees, activities and expense items can be used to log time and expenses to the selected project. Check the Core Help for details. So if you have projects that have no assignments, all the employees can see them in the drop-downs and log time and expenses to them.
If you want to control which projects an employee sees in the drop-downs and logs time and expenses to, you should use Employee Control instead as it restricts projects and items that an employee can access. Check the Core Help for details on that. You can also check out this article: Employee Control vs. Project Assignments.