Afew years ago my daughter bought a double bred sonny dee bar mare from a friend, after 2 days I noticed the mare limping on her right front, checked her feet and found a stone, but the mare kept limping, so had the farrier come out, he suggested calling the vet before he trimed her ( thought it may be her shoulder) had the vet come out and without even touching the mare, the vet said she was foundered all the way around,(no way) and drew blood the mare was about 2 mos. From foaling, anyway she did a pelvic and said the mare was not bred, I told her I 've seen the foal move, she said she must have dropped the foal, this was in sept.
Well, I called the friend she bought the horse from and she agreed the mare was bred and we called her vet, we treated her twice for Lymes with no inprovement..
This mare did the same thing as your mule only she used her tip toes, and would pivot when she needed to turn once up.
We then called another farrier, and he agreed to take the horse to his place, after 6 mos. She was fine( she did foal also and was not foundered )long story short, he said she was not balanced, and had been trimed incorrectly over the years,he still has the mare and his wife shows the mare.