Okay, I think I have solved this once and for all. I'm not sure if I'm smart for figuring out what was wrong or dumb for doing what I was doing to cause the problem.

I recently developed a practice of opening each forum (ORG, Sun Deck, Tavern, etc.) in its own tab, leaving the Main forum page open in the original tab. Then I would click the "Mark Forums Read" link on the Main forum page and close that tab before I ever started reading the individual forums. My thinking was that the forum software would apply whatever cookie settings I had at the time the individual forums were opened in their own tab.

Apparently, however, it doesn't work that way. Instead, the forum software checks the cookie settings at the moment of the thread click, not at the time the tab was opened. Therefore, once I clicked "Mark Forums Read" in the original tabbed window. my cookies were set to that, and all successive clicks took me to the end of the thread, even if those clicks were occurring in other tabs.

I hope that makes sense to anyone who cares.