shadowlastchange not updated automatically by chauthtok

I am successfully changing a password using chauthtok.

However this does not cause shadowlastchange to be updated.

passwd works and changes the date correctly.
I've looked at the source of passwd.c and I can't see anything obvious that achieves it there that's done differently.
do_pam_passwd is called, which effectively just does:
ret = pam_chauthtok (pamh, flags);

What am I missing that might affect this?
Is pam_chauthtok responsible for updating, or delegating the updating, of the shadowlastchange?
I figure it must be, since that's all passwd calls (it exits aferward do_pam_passwd).

Thanks

Gareth