Why might pam not use unix_chkpwd?

[Paul Howarth]

I've seen a few reports that proftpd's sftp support isn't working with
SELinux in enforcing mode:

https://bugzilla.redhat.com/show_bug.cgi?id=1529576
https://github.com/proftpd/proftpd/issues/659

Using strace, it appears that proftpd is rejecting logins after failing
to access /etc/shadow, but why would it be doing that at all, rather
than using the unix_chkpwd helper?

Googling this, the only similar issue I saw was this:
http://blog.siphos.be/2014/12/why-does-it-access-etcshadow/
but this seems to be different because ftpd policy does include
auth_use_pam.

Any thoughts on this? I did try this locally and couldn't reproduce it,
so it seems to be configuration/environment-specific rather than
something being fundamentally wrong.

Paul.