If you have a custom Forum permission defined as an integer and attempt to override the user group permission in Node permissions by setting that number to zero, the value is not saved and it is set back to inherit.
In this context, 0 means inherit so that's not entirely unexpected.
Even so, you'd be unable to override a more permissive integer value (e.g. a higher number or unlimited) with a less permissive integer value. So if it's set to 5 on the user group, and you try to set it to 4, 3, 2 or 1 on the node for that user group, then the most permissive value (5) will be the one that takes effect. Similarly, 0 wouldn't actually mean 0 in this context, but it would just mean "Inherit" so that would also resolve to "5".