I recently started using sharepoint online. I faced a problem while sending emails through sharepoint designer workflow. But for some reason the alerts in the document library and/lists in the sharepoint online are working.
You can check the email address updated in the user profile for that particular user. To isolate the issue you can use write to history option to write the user email ID in the history list. This way you can identify what email address is fetched by Workflow for sending an email and is it same as in UPA.