The buns are from the exact same batch of dough. Proofing time was different.
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former Community Editor
Are these the baked loaves? I would guess that it has more to due with potential hot spots in your oven, here's how to find them: https://food52.com/blog...
If anything, overproofed dough should have darker spots where the gas bubbles near the surface have made the crust extra thin and they cook faster and then darken earlier. Are these egg washed with white/yolk/water/salt? That could help darken them a bit. And yes, definitely rotate your pans half way through the bake and switch racks top to bottom in a home oven for the best temperature consistency. I have 3 dozen brioche buns in the oven right now with pearled sugar topping. Time to take them out! Bake on.