Battery icon shows when phone is charging while powered off
I am using a iPhone 7 Plus, and every time when I charge my phone when it’s already powered off, a battery icon appears on the black screen. The phone is charging because the battery is gets full after a while. The only issue is that I can’t get a black screen/ the screen doesn't turn off when my phone is charging while powered off. Any ideas how to fix this? Please help me! Thank you.
iPhone 7 Plus, iOS 15