Bread crust going soft on cooling?

I’ve been baking loaves at home recently and they are baking beautifully, hard crust, lovely dome, perfect internal texture. However on cooling the centre of my loaf (the crust) dimples and goes soft. The insides are definitely cooked and the loaf isn’t over proofed, I’ve done the poke test. I’m cooking with steam for the first 10-15 minutes. What is going wrong?