1 hen started sleeping alone.
I have one who was raised alone by a broody, and has always tended to keep to herself. She's about two years old now. She now sleeps on the roost with the rest, but for most of her life she would find one spot or another in the coop where she would sleep by herself. My flock of 10 tends to stay in 3 groups as well, though they share one roost. It's quite normal for them to group themselves like this. As long as she's able to get food and drink, and not being viciously attacked, I'd just let them all do their thing.
Happiness is when you can look at your chickens and smile....
Worry is interest paid on trouble before it comes due.
Flockwatching my little bunch of mutts, a favorite pastime.