Alright, I spoke with the developer that rebuilt the interface and learned that the QT backend we use to build our software was updated from QT 5.6 to QT 5.12. The reason this matters is that macOS support is limited to the three latest versions as stated here in their documentation:
This is something that was not included in the changelog because our entire build environment here at Topaz Labs has been upgraded from QT 5.6 to QT 5.12. Everything we release going forward will have this requirement, it appears. This is part of the reason we're not able to support too many OS versions, as we cannot maintain multiple build environments with such a small team.
This is definitely an oversight on our end, and it looks like A.I. Gigapixel 2 will be the last version of this product that Mac OS X 10.11 users can use without upgrading to macOS 10.12. I'm going to speak with the team about this change and update our documentation accordingly. I'm not 100% sure on this, yet, but if this build environment change impacts all future updates, more products may be unable to launch for Mac OS X 10.11 users as they are updated with our new toolchain. The reason we made the upgrade is that QT 5.6 had many operational, performance, and system-level bugs that were plaguing our own build process and our end result products. We had to make the change eventually, and as Mac OS X support is gradually dropped, it was inevitable that this change would occur. We don't actually have any more machines in the office running 10.11, so that's another reason this was missed.
Sorry for the trouble, those of you on 10.11, but it looks like this change will be permanent.