RE: Creating a system dashboard in the classic designer not appearing on the front end UI
Hi,
There are two different types of system dashboards. One is created for entity and the other is created for the whole.
For example, I created a dashboard for the Account entity called Dashboard 01.

And I created a dashboard not only for a certain entity called Dashboard 02.

When opening the App Designer in Power Apps, only Dashboard 02 can be found in the list of artifacts Dashboards.

Dashboard 01 can be found in the list of ENTITY ASSETS Dashboards. (Here is in the list of Account Dashboards.)

However, both of them need to be added into the App Designer and published if you want them to be shown in the application.
It is not the change with the newest release. It is a design existing in the old version.
To know more about the difference, please refer to the following documentation:
Add or edit app components with Dynamics 365 Customer Engagement (on-premises) | Microsoft Docs
If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya