Yes. They are old enough to do that at 1 day old. And it isn't like they are going to die from hypothermia being on the "unheated" side of the brooder inside a house.You think by 5 days they are old enough to make it back to the heat lamp without direction?
This is at nearly 1/2 their expected lifespan. That chick looks huge for 5 days old. It could very well be due to excessive growth rate. Waiting to restrict at 4 weeks is to maximize feed to meat conversion in the shortest amount of time hoping to minimize losses. You have one struggling already so I would do what I could to slow down their growth rate and that can only be accomplished by not letting them eat all they want. They need restriction.restriction around 4 weeks
If you are uncomfortable with the 12/12, start with 16 on/8 off for a few days while you ease them into more of a restriction. But they shouldn't be able to eat all day and all night. With a heat lamp with no true day/night cycle, they could do that.
Have you gotten a few drops of nutri-drench into that chick yet? Any response? It is also possible the little thing is just a FTT. Losing 1 out of 50 to that would be incredibly good odds.