If you didn't read the version of the page just before saving your edit (e.g., if Harej's changes were saved while you already had the edit window open, or if you clicked the edit button from the history page without reading it), then it will mark them as not having been read, so that you won't overlook the older changes. This is probably normal and expected, as well as being desirable.

I have just reproduced this bug on frwiki with the following steps:

1. Visit History page of an article with three new diffs since last visit.
2. Open in a new browser tab the older diff of the three ones.
3. From the diff tab, open the next diff in a new browser tab (“newer edit” link)
4. From the lastly open tab, open the next diff in a new browser tab again. So, 4 wiki tabs are open: History tab and 3 Diff tabs.
5. From History tab, open VisualEditor clicking Edit link, then perform an edit.
6. Click on History: your revision and 3 (or 2, I’m not sure) last revisions you had reviewed in new tabs are still marked as “updated since your last visit”.

A kind of regression of T43759: Viewing old revisions and diffs should reset "Updated since last visit" markers only to that point in page history?
Just a rare caching issue?

EDIT: in fact I am just seeing this bug with another page which I have not edited. So this is not related with editing.

I’ve seen this bug on MetaWiki and on fr.Wikipedia.

But this is probably a duplicate of T218511: After opening a diff, entry on Special:Watchlist sometimes stays unread (bold).