Ok, understood. the issue is how the permission is being checked. The permission is checked on the topic but not on category. Hence when you are using category level permission, it is not working. Please follow the below steps.
Edit plugins/cjforum/topics/topics.php and find below code at line 161
If you are going to release this as a fix in the next version - i will wait for next version. Till then I am happy to set the permission as Allowed or Denied instead of inherited.
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