How to Resolve Corruption Detected by BACKUP WITH CHECKSUM, But NOT DBCC CHECKDB?
I'm not sure how to fix this


In the comments we investigated but never found out conclusively whether the page is allocated or not.

If it's allocated this is definitely a corruption that CHECKDB missed. A bug in CHECKDB. Please report that to Microsoft.

If it's not allocated here's how I would fix it: Fill the file with dummy data  e.g. CONVERT(binary(8000), 0x). Now the page should have been consumed by the dummy data and overwritten with something clean.

Actually, you can use this idea to test whether SQL Server thinks the page is allocated. If this works it's definitely not allocated.

Alternatively, you could use the write page feature to zero fill the page but that leaves an audit trace in the database boot page. Microsoft support will not like finding this, I guess, when they help you in the future with something. They might think your page write caused damage. Also, it steals all the fun from this investigation by terminating it without resolution :)