After multiple Restore and Factory Restores and a session online with Apple Support that resulted in "put an Apple-branded battery in it", I have come to the conclusion that the 11.3 beta for iPhone 6 is incompatible with non-Apple-branded batteries.
The result: my iPhone 6 is now worthless until I can get an Apple-branded battery installed. Right now there's a 5-week queue in the Portland area for an iP6 battery.
My battery life was poor (6 hours) so I knew it was time for new battery. Running 11.2.5 at the time, Apple announces battery replacement program. Immediately Apple batteries are at a premium and I can't get one. So I ordered an iFixit battery and installed. New battery, new lease on battery life. Wonderful.
Then I install 11.3 and suddenly the phone is shutting down with 30-70% charge remaining indicated. When I plug into external source, phone restarts and battery shows the same 30-70% charge level within one minute of booting up. Upon boot it shows 10% range. No Li-ion battery charges that fast without heating up dramatically. If I immediately unplug external source, phone shuts down again in a few minutes. If I let it charge to 100%, it runs for a couple of hours then shuts down somewhere in the 30-70% range. 11.3 beta 2, no difference - except the battery health management function says battery at "maximum performance" - as a new battery should...
So, Restore to 11.3 beta 2, no change. Factory Restore to 11.2.5, no change. Apple Support does diagnostic on battery life over internet, no issues. Then they tell me "put an Apple battery in it." Not "we know there's a conflict with non-Apple batteries", just "put an Apple battery in it." Which, of course, has a 5-week queue.
One of two obvious conclusions: either Apple knew 11.3 battery health management function doesn't support non-Apple batteries and pushed out the beta program anyway, or Apple didn't test the beta program for non-Apple batteries.
Either way, I have a brick iP6. One obvious result: I'm done with the Apple Beta program for iOS.