Is it just me that is getting an error when opening watched threads?
Normally when you click on a watched thread it takes you to the unread post.
On old watched threads it takes you to the first post (which it always has).
As I navigate MR by watched threads quite a bit it's a bit annoying.
I'm seeing this on Safari on my iMac and iPhone 7+
What Safari version are you using?
You're not alone. I just got back to my regular computer after a couple weeks away, and it's not working the way it used to. As you said, it would take you to the first unread post. Now it just takes me to the top of the page that the first unread post is on. I have to click the "Go to First Unread" button to get to the unread post. The button works, but it didn't need to be used before.
Oddly, the Firefox I have on the same computer doesn't have this problem. It goes to the first unread post like it did before.
EDIT: EXAMPLE
I read this thread, which I've been reading since it started:
https://forums.macrumors.com/threads/what-is-was-for-dinner.2133151/
I clicked the thread item in the forum listing, and it ended up at this URL:
https://forums.macrumors.com/threads/what-is-was-for-dinner.2133151/page-2
However, it briefly showed this URL in Safari's address bar while the page was loading, and before the unadorned "page-2" URL appeared:
https://forums.macrumors.com/threads/what-is-was-for-dinner.2133151/page-2#post-26405183
Safari displayed the top of page-2, with the "Go to First Unread" button available. I clicked it, and it then went to:
https://forums.macrumors.com/threads/what-is-was-for-dinner.2133151/page-2#post-26405183
So it looks like something is going awry when the page is loading, because it shouldn't be losing the "#post-26405183" fragment, only to have it put back on with the "Go to First Unread" button.
And now after adding the EXAMPLE, I tested each of the links by clicking on them, and none of the ones with the "#post-26405183" fragment work in Safari. They only go to "page-2", not to the post on the page.