When I create a new project from an existing template, the Start and Finish dates are not copied from the template. Instead, the new project defaults to today's date for both.
This is also observed when creating a new Idea from template.
The same behavior happens when creating from Classic or MUX.
Release : 16.0.2
Component : Clarity MUX UI Usability
The "New from template" is not a 1:1 copy of a project.
When you are creating a project, it is normally the moment where you already know the new dates for the new project (normally, not all projects have the same dates) and you get the opportunity to set them.
In the Modern UX modal dialog you can also specify the new dates before creating it, and other items from the template that depend on these dates (like tasks, cost plans, etc.) will be adjusted accordingly.
If a template has tasks as well, the finish date will move out to accommodate the duration of the project/tasks.