User maintained unplanned substitute, Generally emails need to trigger when substitute switched "ON" for unplanned substitute. Here our issue is emails are triggering even substitute is "Not switched ON" by substitute.
Maybe it is a misunderstanding of the use of the Activation of the substitute. It is also not clear if your user has maintained the substitution in "Workflow Administration" app or in the My Inbox app (please clarify)
Please see the help entry here:
Unplanned: The substitute can take over to see also tasks of the principal.
Unplanned (active): The substitute has taken over the substitution.
Seems the substitute has visibility on the owner's tasks and can optionally act in the "Not Active" setting. But they officially take over on the "Active" setting. So in both cases, to me it implies the emails will go out.
If you delete the unplanned substitute, do the emails stop? seems like answer should be yes.
Maybe you can set the Planned substitute- but it is not clear if a user can set a planned substitute for an absent user (thus the unplanned option).
This thread may also help: https://answers.sap.com/questions/13767771/set-substitution-on-behalf-of-another-user.html
Here is the help for substitutions in My Inbox: https://help.sap.com/docs/SAP_S4HANA_CLOUD/0e602d466b99490187fcbb30d1dc897c/2c950e3d77504dd2a5dde48c...
"You will see the list of users who have nominated you as their unplanned substitute. By default, you will not be receiving their tasks. If you want to take over, you will need to activate the substitution for that particular user."
If you see the product not acting according to the help after fully testing, please log a support ticket.
Hope this helps