Why is absence going to line manager's manager instead of own manager?

We have one employee whose absence requests are being incorrectly routed to their line manager's manager instead of their own line manager. Their line manager is correct in the system. There are no delegations. There are no future changes for anyone involved. I ran the Refresh Manager Hierarchy process. It doesn't matter if the absence is submitted today or far in the future. And it's replicable in a lower environment. This is the only employee we've seen this happening to.

How do we fix this? Why is it happening?