I have finally updated to Anniversary (directly .67), so I have tested the bug myself.
I am overall pretty happy with my improved Ativ experience, I suggest the update.
I used the register key values of Lumia 640 XL 3G to get into the updated ring (using the register editor "roottool.xap"). Then I had to make hard reset because of an issue with the audio volume, that had actually started before the update. No problem with cameras so far. Along with the volume, the update fixed the app list (it was showing some uninstalled apps before) and finally the notifications in my band 2!
The AU notification improvements are very welcome, and the better overall performance as well. The battery life in particular is amazing. I have realized how 10586 was terrible for energy consumption, I have even bought a new battery recently... Now also my 3 years old battery does it (almost) the whole day!
Regarding the brightness, here is my experience so far:
the automatic option does not work at all, the brightness keeps constant regardless of the lux sensor. So I disabled it, you know, manual adjustment has also some pros after all. The problem is when you approach to the minimum brightness level. If you go below 4% the screen turns off, but not the touch panel. The easy trick is to remember the point of the screen to touch for a "safer" and higher brightness level. Then the lock screen will appear. The phone does not interpret a low brightness anymore (honestly was comfortable in the darkness): even with low slider positions the minimum allowed level is 25%. Since the action toggle is still usable after all, this bug is not such a big deal. The method suggested by Tourniquet could be a good solution though.
Happy Ativ and happy summer everyone!
EDIT: The brightness happens to show "random" behaviour sometimes (e.g. unresponsive toggle) ... And yes, also my APN resets after every reboot, unfortunately.