I haven't observed that changing a thread's Watched status has any effect. The only thing that I have seen have any effect is the "Mark Read" button on the forum the thread is in.
I've used the Mark Read button on individual threads numerous times and the issue reappears right away, or within minutes. I've also used the Mark Forums Read option numerous times and the issue persists.
As someone above me stated, I'm also getting multiple threads getting stuck in this unread status. It will get hung up on a particular post, usually one of mine, from days or weeks ago, and when clicking on on the thread it will take me to that older post and then I have to scroll down to the real newest post since my last visit, even if that means clicking through a page or two.
I've tried several browsers, Firefox-based and Chromium-based, on Linux, Android, iOS, and macOS. Fresh and clean installs of browsers and the issue replicates itself on all of them.
This
thread is one of my worst offenders. I have it unwatched and no matter how many times I read to the end of it, or mark it read, it stays stuck on unread upon returning to the Community forum and clicking on the thread brings me back to the post as linked above here.
On the main
https://forums.macrumors.com/ page, the default filter is to show unread threads only. As we can see the thread example above is visible in what looks like a read state. Though because it was looked at, it shouldn't be showing up in its current state and page filter.
Has
@arn looked at what this issue could be yet?