Same issue here with an iPhone XS Max.
Unfortunately, it was not charged over night and went down to 2%. After plugging it in, I realized it was not charging. The phone did register it was plugged in, meaning the little lightening bolt appeared over the battery. However, on the lock screen, it said "Charged" instead of "Charging", even though it was still at 2%. When I attempted to restart the phone, it would not turn back on, instead giving me the low battery icon.
I understand when the battery gets very low, the phone needs several minutes of juice to get the battery above a certain percentage before turning on. Since the phone now refuses to go above 2% (or at least the phone THINKS it's only at 2%, who knows?) it won't turn on until it meets that threshold. No amount of charging (using various cords, boxes, wireless charing pads, directly into my MacBook) will get the phone to turn on. This means I can't restore the device to a previous backup/iOS.
I have now left it either plugged in or on a wireless charging pad for several hours to no avail. I submitted a Feedback Report (FB6641819) but haven't heard back yet. I contacted support, but their only recommendation is taking the phone to a store, the earliest-available appointment being Tuesday afternoon. Although he did assure me that if the techinician did not find anything wrong with the device, there would presumably be no charge for the repair/replacement, since the fault is with the software.
I've been part of the Public Beta program since iOS 6 and have never experieniced such a terminal problem. I'm now stuck with an unusable device for the next several days. It's quite disappointing Apple would release a Beta with such an unstable build to the public.