Steps to reproduce:
1. Start the portal (used Liferay 7.1 dxp-7 + hotfix-2854-7110 for the test)
2. Enable Single Approver workflow for Web Content Articles
3. Create a user
4. Add Portal Content Reviewer Role for the user
5. With a different browser, login with the user (to make sure it has been activated)
6. As the default admin user, Create a Web Content Article and send it for review
7. Check the notifications with the other user
Checkpoint: User gets a notification about the review task correctly
8. Edit the Single Approver Workflow, and without any modification, save it again
9. Once again, as the default admin user, Create a Web Content Article and send it for review
10. Check the notifications with the other user
Expected Results: User gets a notification about the review task correctly
Actual Results: User doesn't get a notification about the review task
Most likely the Workflow Designer modifies something in the Workflow when you save it.
Customer's use case is a bit more complex: They are using Page Versioning with Workflow and would like to get Notifications for that task. Since the behavior is the same with Web Contents, I simplified this. Customer also uses a different Role for the workflow task assignment, not the default Portal Content Reviewer.