Sorry about that misinformation. My brain misfired and I was confusing MG with infectious bronchitis virus in my memory, which isn't all that trustworthy these days.
Has this chicken had an mild respiratory symptoms over the past few weeks or so? If so, it may be Mycoplasma synoviae. It's a corona virus sort of like Covid 19, where the real damage occurs in the internal organs. One of the symptoms is swelling in the lower legs and feet. You would need an...