Thanks. The arrangement looked ok today, but set the floating threads to on.

(Pedro, I recall inquiring about this some time ago, and the issue seemed to have resolved itself, but now it's back.)

It used to be when I logged in, whatever the latest entry, whether a new topic or a reply, that thread would go to the top of the Main Forum when returning to the Main Forum. In other words, the sort order "rule" was the same, logged in or not. Lately, after logging in to post a response, the thread drops down somewhere that may even require doing a Search to find again. After posting, and going back to the main thread (still logged in) the thread is still somewhere else; not at the top.
I've tried various options in the Control Center with no effect, other than what their intent is.

Any tips as to what needs to be reset? Could it be a setting or settings on my Mac?

As I rarely post via my iPhone, I can't readily say if the problem occurs there, too.

At the top of the thread list, do you have floating threads (next to RSS) turned on?

I don't think I ever changed that setting. Now to see how it goes.