I think that is normal behaviour. My iPhone 11 Pro Max has done it since I got the device new last Nov. It concerned me at first, I kinda panicked thinking it wasn’t going to turn on. Just turns out it needs more time to finish closing down.
I do remember reading another post about it, but that was so long ago no way can I remember when.
But to puts your mind at rest, I’ve not had an issue with it at all.