I [[ https://fi.wikipedia.org/wiki/Wikipedia:Kahvihuone_%28tekniikka%29#.22Merkitse_kaikki_sivut_n.C3.A4hdyiksi.22_ei_merkitse_kaikkia_sivuja_n.C3.A4hdyiksi | reported ]] this on fiwiki over a week ago. It's happened to me only on fiwiki.
So, how it goes for me:
1. Go to check your wathclist
2. When there are some unvisited pages (showing bolded) click "mark all pages as read"
3. Those bolded pages are still bold, but if you refresh the page after it, they are not bold anymore.
This only happens sometimes, not even every day. I'm using Firefox (the newest version)
**I think that number of pages on my watchlist has something to do with this. Currently I have 9929 pages there.**
Also another user on fiwiki wrote that he/she has the same issue. He told s/he has 9282 pages on their watchlist.
Btw, this seems to be the same as many years ago reported T9542.