Always best to try & get the most complete history as possible, so the antibiotics were duly noted.
So unfortunately, from afar, all I can do is venture some guesses.
First is the possibility of skin mites, even with your other pig not showing any signs of being itchy. Did the vet do any skin scrapes? If not, you might ask about just treating both pigs with a topical like selamectin ( Revolution ) or Advocate.
Next is the possibility of barbering by either pig. This can occur when there is less fiber in the diet, or even from stress. Make sure there is plenty of fresh hay for them to much on in addition to their pellets & fresh veggies.
And, try just washing the fleece in hot water, no detergents & see if this makes a difference.
If the wounds have scabbed & seem to be healing, the scrub may be doing its job. You may never discover the cause & it may just be that the lesions will heal on their own, but I would still consider treating both of them with a topical just in case.
There are some uncommon causes of skin lesions in pigs, including endocrine imbalances due to cystic ovaries, & some cancers.
Your pictures just came through. Give me a few minutes to review.