Brooder is about 2’6” x 3, heater is in one corner,. Plenty of room to escape the heat, but they only decide to shower at sunset. Thinking they want to get “under” something at sunset.
I tend to let my chicks behavior lead me. Moving away from the heat indicates to warm.
Your brooder is pretty small for a dozen nearly 2 week old chicks. Maybe that is the farthest they can get from the heat.
Generally mine are fully off added heat in spring by 4 weeks old. The temps range from 40's at night to mid 70's during the day when I raise chicks.