Hi!
If I understand correctly, the ones eight weeks old are living in the run temporarily? So that shouldn't have anything to do with why they're sleeping in the run, unless they like keeping an eye on them, right? I'm kidding there, as I highly doubt that's it.
Some reasons chickens won't sleep in their coop are overcrowding, something scared them in there, they like the perches better in the run, the coop is too hot for them, etc.
Each chicken should have 4 square feet in a coop. If they have that, then you can scratch overcrowding off.
Could there have been a stranger in the coop recently? If not, scratch that off.
How about the perches? Is there enough at different heights in the coop?
The only time we've had issues here with that, was younger ones wanting to sleep outside. They'd go in and out of their shed/coop all day long, but wind up out in the pen at night. When this was going on, our evenings were in the 60s, so after a week of this, I sprayed them lightly with the water hose. That did it for two nights. Then back to it again. Now it's too cold to try my trick again as they're only two months old. If they were adults, I'd do it again. So now we put a little led light in there, and that did it. They go in.