3184170 - Why am I unable to add or modify a user's supervisor?

I am unable to add or modify a supervisor for a user, and no error is shown anywhere:

• In the user interface, the desired supervisor cannot be assigned, or the change appears to save but does not take effect. No error message is displayed.
• Importing the supervisor through a data import file does not apply the change, yet the import task completes and shows no error in the import error history for the affected user.
• The affected user's supervisor stays at its previous value, and the user's last-modified date does not advance even though the import file is loaded on every run.
• The same user fails to update on every attempt, starting from a specific date.

KBA , BNS-ARI-CP-CORE-UM , Core Administration User Management , BNS-ARI-CP-CORE-DEL , Delegations , BNS-ARI-CP-CORE-IEX , Core Administration Import/Export , Problem