Chad Callihan fixes a mismatch:
Just as you can’t judge a book by its cover, you can’t judge a SQL Server login by its name.
With availability groups, you may notice a login on a primary replica and a “matching” login on the secondary replica. But just because the login is named the same on each replica doesn’t mean they’re the same to SQL Server.
Click through for the explanation and how you can fix it. The other way to fix it is to use Active Directory (or Entra ID) logins, though that’s not always in the cards.