For some reason, it seems that whichever method is tracking the lastread status of a topic - is not working properly...
I keep seeing 'unread' status on topics that are 3 and 4 days past and have been read, and can even set "Mark all as read" and within 24 hours, the old topics are re-appearing as un-read...
Just an FYI - as I know changes have been in the works on this issue --
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