If I set a member with below groups assigned:

Group primary T

Group secundary X

Group secundary Y

 

What happens if a forum permissions as below:

Can see by Group secundary X

But can't see by Group primary T and Group secundary Y

Then that member can see X if it's working as designed.

In the IPS Guides section it says:

The primary group is what displays under their name but they also inherit the permissions of any secondary groups they are in.

So looks like the "no permissions" in that case get overruled.

Permissions are additive not subtractive. So if one group gives you more you get the perks of the "more".