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If the user was from onprem that explains it. I think the attribute 1601-01-01T00:00:00Z is typically shown when no actual data is stored in Entra for that field.
If the password is updated for that user you will likely see the recent value.
If the Entra Connect sync is still enabled, check that password synchronization is corectly setup. Also is this only one particvular user or more than one user?