The user has a HR Case. Attached to the HR Case is a workflow. Within that workflow, there is an Approval activity which is dynamically calling the assigned_to user's manager. It seems that the user's profile is taking a manager value other than the one specified in their sn_hr_core_profile record.
The issue is that the user in question "Bucky Barnes" has two HR profiles, and the inactive one was selected for the "Colleague" field, which had "Helmut Zemo" selected as the manager.
When the HR Case was raised, the inactive profile of user "Bucky Barnes" was selected:
This is why the manager being selected to send notifications to is Helmut Zemo, and not Steve Rogers.
Bucky Barnes' other, active HR Profile shows Steve Rogers, but this is not the profile that was selected:
The workflow, therefore, did exactly as it was told based on the information it was given.
It was recommended that the user add a simple reference qualifier to the sn_hr_core_profile to weed out any profile that is not active = true, thereby preventing this behavior from happening again.
Here is some documentation on reference qualifiers: