Whatchlist bug

I've noticed the following behavior when I go to [[Special:watchlist]]: when there are multiple changes to a page in my whatchlist, and I had visited the page earlier, the whatchlist has all changes to that page marked with

<strong class="mw-watched">...<strong>

, even those which I've already checked out earlier.

See the screenshots.

The [[Special:watchlist]] should have

<strong class="mw-watched">...<strong>

only for the edits updated since my last visit (in the screenshot, only the first).

Version: 1.20.x
Severity: normal

Attached: