I’m not sure whether you’ll be able to replicate this misbehavior, but here is a description of what I’m seeing anyway…
When I click on “forums” at the top of the Haiku homepage (directly across from the Haiku logotype), I am taken to the Forum start page and it looks fine.
But when I click on “Forums” in the drop-down list box under “Experience”, I am taken to the Forum start page where only the major category headers appear; no specific forums or posts are listed, and navigation deeper into the forums is precluded.
Clicking next on “Forum Index” in the main navigation bar across the top of the Forum start page causes a refresh that brings up all the missing content. Then I am good to go again.