Ok, this is a consistently confusing issue, and unfortunately I am not immune to the confusion! I will try to state how I understand it, and hopefully Matt can correct me as it will almost certainly not be 100% accurate.
Here's the thing: As I understand it, Ray Detail Region does NOT affect GI! From the results I've seen, it seems to have consistently not addressed GI-related issues like this. And my recollection from multiple posts from Matt also indicates this, though I'm not sure it has ever been crystal clear.
I know that the feature was originally introduced to address *shadow* issues with off-screen geometry (perhaps among other issues, e.g. reflections from off-screen geometry). It would also help with shadows from out-of-crop geometry with the right settings, of course. So this was its purpose. But that does not say anything about GI...
I think what happened is that, because it helped with off-screen geometry shadows, people assumed it did the same for GI (for everything basically). But again I don't think that's actually the case. That being said it would be good if it did do that. I'll have to have Matt confirm/deny/clarify all this.
- Oshyan