Interestingly I have seen a few examples of this recently.
LastLogonTimestamp is an attribute that is replicated between domain controllers, however it is only updated every 10 to 14 days by default
Have you checked individual domain controllers to see what the lastlogon attribute is set to. One of them will show the time the user actually last logged on.
I’m not convinced that lastlogontimestamp is actually reporting the correct time of last logon.
Check if the msDS-LogonTimeSyncInterval of the domain controller has been modified to a longer than expected value