I have this same issue popping up with lots of my borons. After I flash any code (including going back to Tinker) when the device restarts or I unplug USB and plug it back in the main RGB LED flashes white once and then the device turns off.
Reset does nothing, all I can do is hold the mode button down and get them into DFU.
With a battery connected it makes it further, tries to connect flashing green for a couple seconds, then resets and flashes white, and starts over.
We want to switch our product from electrons to borons, but keep running into issues.