Enable distinct forum permissions

I was trying to create a new forum where unregistered users are able to post (including captcha check) - i have setted up all forums with the intended permissions for all levels but the forum that should be enabled for guests & public to post does not show the "new topic" option if the user is not logged in. For me it looks like the enable distinct forum permissions do not have any effect?
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did i miss something?
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kind regards