Mark forums read issue?

[Black Tiger]

I'm busy with a SMF 2.0.9 forum and something just caught my attention.

If I click the "mark all messsages read" line on the forum index. All messages are marked read, except for the subforums.
On all subforums the "new" message icon is still up.

Did I forget a setting, or is this a bug in 2.0.9?
And if it's a 2.0.9 issue, how can I fix it?

[Black Tiger]

Solved. Probably caused by old cookies or something else after conversion.
After clicking "show new posts" and then clicking several times at the "mark forums read" suddenly all was working as designed.
So probably cookie issue.