Oops. I am very sorry for wasting your guys' time. I now found the problem: the test org file somehow contains a line somewhere (I guess it was automatically generated by org-mode when I first created the file) that overwrites html- postamble to be auto.
- [O] Remove postamble in exported html from org-mode Xin
- Re: [O] Remove postamble in exported html from org-mode Nick Dokos
- Re: [O] Remove postamble in exported html from org-mo... Xin
- Re: [O] Remove postamble in exported html from or... Rasmus
- Re: [O] Remove postamble in exported html fro... Xin
- Re: [O] Remove postamble in exported htm... B.V. Raghav