Strange issue with my iPhone 15 Pro Max and overnight charging.

I usually put my phone on charge each night. It is wirelessly charged with an Anker 7.5 Watt charger.

In the morning when I unlock my iPhone I am told that my passcode is required when my iPhone restarts. I have not powered the device down. The battery charge level is at 89% when the device is taken off the charger. It is usually on charge for around 7.5 hours a night.

Anyone had similar experience?
Malcolm Parfitt