Debugging why KRB5_KTNAME isn't working

I am trying to debug why having KRB5_KTNAME set in the environment of a
process is not actually making that process use that keytab file but
instead is using the default /etc/krb5.keytab.

The process is Postfix's SMTP deamon (smtpd).

I have confirmed that the method of setting the environment variable is
working:

# ps -ef | grep smtpd
postfix  3726845 3515138  0 11:56 ?        00:00:00 smtpd -n smtp -t inet -u -o stress= -s 2
# tr '\0' '\n' < /proc/3726845/environ
KRB5_KTNAME=/etc/postfix/smtp.keytab
KRB5_CLIENT_KTNAME=/etc/postfix/smtp.keytab
KRB5_TRACE=/tmp/smtpd_krb5_trace
GENERATION=55

However when looking at what the process is actually trying to open
with strace, it's clear that it's not using /etc/postfix/smtp.keytab:

# grep keytab /tmp/smtpd.strace6
3726845 openat(AT_FDCWD, "/etc/krb5.keytab", O_RDONLY) = -1 EACCES (Permission denied)
3726845 openat(AT_FDCWD, "/etc/krb5.keytab", O_RDONLY) = -1 EACCES (Permission denied)

As you can see, I did attempt to try to trace the kerberos library with

KRB5_TRACE=/tmp/smtpd_krb5_trace

but that file does not actually get anything written to it:

-rwxrwxrwx. 1 root root 0 Jan 27 11:27 smtpd_krb5_trace

and the above strace doesn't show any sign of trying to open or even
stat the file:

# grep smtpd_krb5_trace /tmp/smtpd.strace6
[nothing]

Kerberos version appears to be MIT 1.18.2.

Any thoughts/ideas?

Cheers,
b.

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