"New" not disappearing after reading thread

Don't know if anyone else has this problem but after reading a thread, which could have had "2new" alongside, it still remains and I have to come out of the forum and in again to delete it. It's quite confusing as I can't tell which threads I have read or whether there have been new posts I haven't read.
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If you live in Derbyshire, as I do.
In the sticks near Peterborough
Glitches always appear on Fridays.
Ill alert the Mods just in case there’s anyone in the office who can tweak a knob or whatever ...
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.