I've had the same random shut-off problem with my Gemini for over a year now.
At first, it seemed to occur only rarely. I didn't take any specific notes, but it seemed to generally happen in conditions with bad radio connectivity, like on a moving train or on a mountain, and only when the device was in active use. It never shut off in my pocket. Every time, the screen just went blank with no warning. Yet, when I started it back up, it would have a high battery level and continue to run normally for hours more.
More recently, the shutoffs became more frequent. Sometimes, after I started it up after the first shutoff, it would happen again as soon as it finished booting and got to the PIN code screen. Sometimes this even repeated 3 or 4 times.
The shutoffs have never happened while the Gemini is plugged into the official USB charger, so it definitely seems like a power issue.
I'm not sure if the reason it's been getting more frequent is the Android 8 upgrade or not. It could simply be the battery wearing out over time, since recently I've also been seeing some normal signs of the battery getting old, such as it showing 10% left and then suddenly jumping to 0% and showing a "powering off" message. To be clear, the low-battery shutoffs are a different issue that I've also had with other phones when the battery gets old. The blank-screen shutoffs are different - the battery level doesn't matter at all, and the shutoff is instant, with no message.
At any case, I got to borrow my friend's almost-unused Gemini a couple of weeks ago. I upgraded it to Android 8, and I've been using it normally ever since. It hasn't shut off once. Either it doesn't have the same bug, or the battery on it is still fresh. At any case, the problem is clearly not caused by Android 8 alone, though it may of course happen more often if Android 8 uses the battery in a different way.
However, the new Gemini does have the disappearing SD card issue mentioned earlier in this thread. I didn't use an SD card with the old Gemini, so I hadn't seen the issue before, and don't know if it would have happened with Android 7.