In the documentation for using application aware processing on SQL, there is nowhere that states that the guest account (svc.veeam in our case) has to have any kind of special rights inside SQL whatsoever. I am under the impression that the network service or NT Authority\System account handles that on the SQL side via VSS.
On some of our SQL servers, that account, does in fact have a login and role in SQL, and everything is working on them just fine, even after patch. On the ones that don't, the backup portion still works, but the Log backup/truncation does not.
Here's the day before I applied the patch, and as you can see, the login error for svc.veeam still exists, however both backup of DB and Logs happen just fine.

Here's the day after, and as you can see, the login error for svc.veeam still exists, however ONLY the backup of the DB happens.

What in the code changed that would make it a requirement for this account to have this kind of access, and why did/does it work for the db backup just fine even after giving the same message?