Since the iOS 13 update, I'd had this happen almost daily:

The battery level is reported as fine and everything works like normal from 100% to about 35%. At about 35% (could be 36 or 32 or whatever any given time--I can't see that the exact percentage matters), the phone crashes to powered off state. If I try to start it up with the power button, I get the "plug this in" indicator. If I start it using the hard reset method, then it'll power back up normally, and the battery will always report as 10% at this stage. It can stay like this for a few minutes up to a couple of hours, but it'll inevitably crash again if I don't plug it in. After it crashes, I can do a hard reset again and it'll come back up and the battery will often show something percentage in the 20's. Often if I try to use any app in this cycle, that'll cause a crash. If I do plug it in after a second crash, then suddenly the battery is reporting at maybe 23% again. Like, WTF?

It only started happening since iOS 13.

I would be happy if my iphone only lost most of its battery life. :)

That’s the new setting of the battery where the software fakes 100% at 90-80% of capacity based on the age and performance of the battery.

This can be turned off in the preferences.