I am trying to get additional info about why this error occurs when
I call FltSetInformationFile for FileValidDataInformation on a
particular file. It is a file my driver just created. And it is the
only file this occurs on (there are over 10K files created by us in
the same situation and only one fails a VDL change).

I know the only possible situations are memory mapped file (I did
not check, but considering we are creating the file, and do not allow
access to it, I do not see how …) and security (not the case because
we can change VDL for other files in the same thread).

Anything I missed?

Is it NTFS? If so you can use the status trapping code. @“Scott_Noone_(OSR)” blogged about it here