I'm using the 'old' (non-JS) watchlist in extended mode and with grouping turned on. For a few weeks now, I've noticed that my visits are often not registered. I.e. I visit my watchlist, then click on an unvisited diff-link or otherwise visit a page, go back to my watchlist and refresh (F5). Often I have to use the button "Mark all pages visited" (Enotif_reset) to actually mark the page I just visited.
And yesterday or a few days ago, it got worse. Now all items for a page shown on the watchlist page (not just in one group) are marked unvisited when the most recent diff is unvisited, and marked visited when the most recent diff is visited, even if previous diffs were not actually visited.