I can't reproduce such behaviour on my BV8000 Pro. I set it to be in "file transfer" mode, and it is always in such mode when i plug it on my PC.
I'd suggest you to flash your devices to the november 2017 firmware followed by a factory reset.
I'm currently using the 20170803 build. I think I'd like to try flashing to see if it will fix the problem. Why do you suggest november 2017 and not the latest one? Also, I have found some tutorials about flashing (seems a bit complicated but think I can handle it?) but still not able to find the best place to get the official firmware. Can you please point me in the right direction?