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 can employees be restricted to log time only in their assigned projects?
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 restrict which employees, activities, and expense items can be used to log time and expenses to the selected project. Check CORE Help Center 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 access management instead (Settings > Access & Permissions > Manage Access), as it restricts projects and items that an employee can access. Check CORE Help Center for details on that. You can also check out this article to learn when to use access management and project assignments.