The bug behaves differently here. Closing Marvin from the multi-tasking bar and re-opening it, correctly preserves brightness at 80%. (I currently have "Asparagus on Black" set up in Night Mode here.)
The wilful drop to around 68% only occurs here, every time, after I put the iPad to sleep and switch it back on. Even an iPad sleep of a couple of seconds will reduce the brightness in Marvin.