Let's investigate your issue by doing the following:
First, you have said the overdue task mail was sent within the last 2 months and it stopped working now! Also, you made sure that the other workflows are sending email properly!
That generally means there is no issue in the pre-configured email setting for your tenant
BTW, Unlike SharePoint OnPrem, you can't manage outgoing email
setting in SPO
Second, you have said the only difference is the
I don't think it causes such issue especially since it was present, and the overdue email was sending properly as you have said!
App step is used to allow the workflow to be authorized with its identity as a Full Control and ignore the current user permissions.
This is will ensure that the workflow will be executed successfully in
case the current user has no permissions. Read more at Workflow was Suspended with Unauthorized HTTP / Elevate Workflow permissions in SharePoint
Third, you have updated your question and said The issue is taking place in Document Libraries, but not Lists!
Start Task Process action doesn't care if it is a list or library? additionally, there is no overdue email setting in both!
Let us sum up the previous points by ignoring the imaginary causes and focus on the real causes.
- This issue is not an email issue.
- This issue is not related to APP Step.
- This issue is not related to if it's library or list!
- This issue is related to the workflow itself by 80%.
Let's focus on the last point,
- Check the
last modified date for workflow, maybe it recently modified!
- Open SharePoint Designer > Workflow > check the modified date column for workflow.
Whether the workflow has been modified or not, try to:
- Check all
Task Process actions, and Make sure that the
task overdue send email is checked and its recurrence settings are set properly as you prefer.
- Save, and Publish.
Don't forget to check also the
Completion Criteria settings, maybe the overdue condition has not been met.
Try to create a new workflow with only
task process action on the current document library and stop the current one to make sure that the issue is not related to the document library.
Also, try to create a new workflow that simulates the current workflow steps on another Document library to make sure that the issue is not related to the workflow.