This could be a possibility. High protein in humans can lead to gout, which can cause swelling in the ankles. Good detective work! I know I give my hens to many treats and BOSS is a big part of that.
Jumping off a high roost can cause bumble foot too. Look for areas where they could injure their foot like a screw head, nail sticking up, rough wood.
I had good luck soaking the affected foot in warm water and Epsom salt for a few days and removing the dark scab. You can then massage the foot...