Symptoms

Task notification email not getting sent to the workflow Originator's manager.
 

Diagnoses

The manager's display name field that was being used for the Task's destination user was not resolving via SmartObject filter. Investigating further revealed that the Manager field attached to a user's SmartObject entry was being created by a Concatenate inline function using ' Last Name], ,First Name]', and for some reason this function was producing a different string than when the name was manually typed in (likely hidden/different characters).
 

Resolution

Copying and pasting the SmartObject value directly from the database into the SmartObject tester allowed the user to resolve correctly, which confirmed that the string was somehow different. Removing the Concatenate function and just placing the control context parts directly (' Last Name], ]First Name]') allowed the expected string to pass through and resolve correctly.