That looks like scabs from minor wounds to me. In the first stage of frostbite, tissue is yellow. The damaged tissue won't start to die and fall off until much after it gets damaged. So its likely not frostbite. My personal opinion.
I do not think that the ventilation is too close to the roosts at all. Think that when your chickens are sleeping at night, they are not standing on the roosts but rather hunched down in a little ball.
Sometimes a tiny bit of frostbite is inevitable. A lot of times its not about moisture or...
At the table. Or if my mom trusts me I can eat at a little tray in front of the TV but that only happens like 3 times a year 😭 One of those times being today so that we can watch the Lions game.😌