I call those spontaneously browned mud lugs "dirt-cured." The combination of sun, heat, and soil moisture allow them to either rot or cure--luck of the draw. In the past, when priming the good mud lugs, I would include those that were half-browned, and they would go with the rest into the shed. Those half-browned leaves never came out as nice as the ones that had not been partly cured in the field.
This year, whenever I saw a half-browned mud lug, I primed it, then laid it carefully on the dirt beneath the plants. I'll go back and collect them when they are "done." It's a perfect opportunity for a good corncob pipe, carried in my pocket. When I see a perfect dirt-cured lug on the ground--one in just the right case, I can knock off the dirt, stem it right there, and crumble it into a pipe. They are nearly always smooth and flavorful, mild and...well, "earthy"--ha!
Bob