Likely unrelated, still linking because similar problem so people can find stuff: T218511: After opening a diff, entry on Special:Watchlist sometimes stays unread (bold)

Thanks, @Michgrig for reporting. I did some additional testing and the issue is present without "Live updates" turned on.

• When Watchlist has a record displayed as Unseen changes (bolded), and the watched page is clicked and opened in a different tab, there will be no changes to the status of the record. The item remains Unseen.
• Refresh the Watchlist page - the record becomes Seen changes (unbolded).

So, the action of the watched page being viewed is stored, but Watchlist page needs additional reload to reflect the new status.

In T265513#6548796, @Etonkovidova wrote:

Thanks, @Michgrig for reporting. I did some additional testing and the issue is present without "Live updates" turned on.

Without Live updates, one has to manually refresh the page to view the changes. That's what Live updates eliminate - the necessity to refresh the page manually.
When Live updates spot a newly edited page, they refresh the page and un-bold the viewed pages. But when there are no new edits, the viewed pages are not un-bolded. That's what the bug is about.