May or may not be a bug but I would expect the following permission setting for forums to function as described below:

When "Can see forum" is set to "0" for a group, I would expect all topics and replies regardless of their settings to not be viewable in "Active Topics" section for both topics and posts for that group.

Using 7.4.1

Also, I would expect when a forum is not marked as "enabled" from the forum settings, it would no longer appear in the Active Topics for topics or posts.

I have not checked but would think same issue for the search function.

There are several posts on this subject. When using active topics or post islands you need to make sure you do not include any private or restricted forums in the list.
I think that is why someone moved this post to non bugs. At this point in time it is not a bug but by design.

I am suggesting this should function differently as should be cascading permissions with highest priority trumping all other permissions. Therefor, when Forum is not enabled, no one has access to it (content island/Active Topics/Main forum/anywhere). If enabled but only Group X has "Can see Forum" set to 1, then all others do not have access to any part:

Priority 1 - Forum Enabled
Priority 2 - Can See Forum
Priority 3 - Can Read Topics
Priority 4 - Can reply to existing topics and Can create new topics


Therefore, forum can be enabled, Group X "Can See Forum" set to 1, "Can Read Topics" set to 1, and "Can reply to existing topics" and "Can create new topics" set to 0. ==> Read Only

With permissions set to Read Only example:
1. I want to disable the forum, so I mark the Forum Enabled so that it is not enabled. Then all Groups should not be able to view it in any location of forum.

2. I wanted Group X "Can See Forum" set to 0 and leave the previous setting "Can Read Topics alone, I would expect Group X not have access to any part of forum.

I should not as admn need to change the lowest priority permission, in this case Can Read Topic, when I higher priority permission is set for no access.