So I have the iPhone 16+, which I bought and set up on December 8, using the side by side method from my iPhone 11. I had my battery down to 7% last night before I plugged it in to charge (using the USB c cable that it came with and a 12V adapter) before turning it off for the night to let it charge. My iPhone 16+ is on iOS 18.2.
This morning I could not turn the phone on, so I had to keep trying using the reboot method - I plugged it into charge further thinking perhaps my charger didn't work, eventually it just randomly turn back on, I couldn't even set up a Genius Bar call since I couldn't turn on my phone for it to receive 2 factor authentication at first.
After looking at the responses here, it seems like:
1) I should not turn off my phone and charge overnight
2) I should get a 20V charger?
3) Set optimized charging to less than 100%
I will try these solutions and see what happens, luckily I'm still in warranty and now I think I need to get the additional one year AppleCare for an extra $200.