I received a response from Odoo Support, and the issue is now resolved.

The problem was caused by a conflict in the specific view that controls the course homepage (website_slides.courses_home) during the upgrade process.

Here is the fix that worked for me:

1. Activate Developer Mode (Go to Settings > Scroll to bottom > Activate Developer Mode).

2. Navigate to Settings > Technical > User Interface > Views.

3. Search for the view key: website_slides.courses_home (or search by ID: 1966).

4. Open the view record.

5. Click the Action button (gear icon or "Action" menu at the top).

6. Select Compare / Reset.

7. Click Reset to file version (Hard Reset) and confirm.

Once I did this and refreshed the public page, all courses appeared immediately.

Hope this helps anyone else facing this regression in v19!