I reported issues about this before which I don't think were issues or bugs regarding the unread view:
One client keeps coming back to me over and over again to keep reporting that threads they claim to have read keep reappearing in this view after 2 weeks or so.
I am trying to disprove this and wondered if there is any suggestions to go about disproving this is or isn't a bug or user error?
At the moment my working theory is the client is having more than 1 user account viewing the URL and its showing posts the other user hasn't read, they do have a single shared moderator account between staff they are supposed to only use for this reason but it is possible they could be logging into another admin account and doing this causing the posts to reappear.
I did think "are the posts being edited? is this causing a new unread indicator to happen?" but it doesn't seem like the threads they say are reappearing in this view are being edited so my only theory to go on at the moment is that its either multiple accounts or is actually a bug (I am inclined to the former)
I am going to make a suggestion to the client to see if we could limit the user able to see this view to just the moderator account to rule this out but if anyone has any other suggestions or has come across possible issues that might explain why threads keep reappearing here other than different user accounts being used that would help me try an rule out any unexpected behaviour here.
wpForo cannot keep read/unread information of all topics for each user for all time. This is tremendous information and will affect the forum and performance. So, wpForo only keeps information (read topics for each user) about last 2-4 weeks. However, this will work fine if you click the [Mark all read] one time a month, this click registers a certain time which says all topics later than that date are read and will never been shown as unread. So you just need to ask your client to click that button one time in 2-4 week. Otherwise, you have to increase the read/unread information period and affect forum performance.