"Post Code Suffix" in Reports is the same as "Postal Code"
You have discovered a bug in the Membership Details report.

The first step in a situation like this is to see whether anyone else has reported the issue on the CiviCRM issue tracker. A link to the issue tracker is at the bottom of every CiviCRM screen.

As of this posting, it doesn't look like anyone has created an issue, so you can write one up. You could pretty much paste in the description you've written in your question here.

To actually get the bug fixed, you may need to engage a CiviCRM service provider.

In the meantime, if you just need to export a CSV of your members' contact information, including postal codes, try using Search > Find Memberships, and in the search results choose the "Export Members" action.