Why does my dog's coat keep changing pattern?

We have a rescue australian cattle dog/red heeler, presumably purebred (we can’t be sure because she was found in the woods but she doesn’t look like anything other than a purebred heeler and the shelter priced her as such). When we first got her it was winter and she had no noticable coat variations, but after her first shed she retained her winter coat along the length of her back and now even when it’s winter she has a noticeable ridge of coarse hair all down her back, but it’s far too wide of a ridge to be a rhodesian ridgeback indicator (it’s about 5 inches wide). She’s retained this for a few years with no other change, but this summer she’s grown leopard spots! Literal, actual shapes of leopard spots along her ribs! I wish I could share a picture! Her spots are a darker brown-red than her coat and we thought maybe they were just dirt but after washing her it only made them more prominent. We thought maybe the hair on her back indicated her being a non purebred, but we can’t think of a single dog breed with spots that are actually shaped like a leopard’s, and everything else about her looks cattle dog. Is there anything that can cause these strange coat changes?