Hi,

Here what happens is, the attendance smart button in the employee form is added by inheriting the original form view in the attendance module. Once the Attendance module is installed, you will see the smart button as it get added and once the attendance module is uninstalled, the inherited view will get deleted  and so you wont see that button again.

For inheriting and making changes in an existing view see this blog:   How to Add Custom Fields to Existing Views in Odoo v12

Thanks