The xf_session_activity table has a field called params. The params field shows what the member is viewing. So for example if the member is viewing a forum called Announcements the params field would show:
The bug that I'm seeing is if the member is on page 1, the params field would show page 2. If the member is on page 2 the params field shows page 3 and so on.
I haven't actually managed to reproduce this, but I suspect it could be related to pre-fetching. We shouldn't update your location for a pre-fetch, though arguably we may just want to disable it entirely (it's added automatically via the page_nav).