It's to do with cookies. The cookie remembers where you were and flags new posts. If it isn't there then it can't. Are you using a cookie cleaner?
You can try Marking all as Read in each forum that ought to reset the cookies, but if it persists after that then somehow your cookies are being ignored/deleted.
(As it happens, mine isn't working right on my PC on Chrome, and hasn't been for months. I'm losing cookies every time I open the browser, yet it isn't configured to do so. All other combinations of hardware and browser are OK)
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