"Could not find login" failures for remote LocalSystem that exists
SQL Server uses SIDs (Security IDs) behind the scenes https://docs.microsoft.com/en-us/sql/relational-databases/system-catalog-views/sys-server-principals-transact-sql?view=sql-server-ver15 

The name column is a label, and doesn't have to match what is in AD as long as the SIDs match. For example, I have seen a user who was able to connect to SQL Server after their AD username changed (married surname), but the old username still showed in SQL Server. 

I suspect that this might be the other way around, and that the name is the same, but the sid was regenerated when the OS was upgraded. One way to test that would be to create login ... from windows for the computer account on a different, test SQL Server instance, and compare the sid column values for old and new logins. 

HTH