White dorsal stripe?

A few months ago I sold a miniature 2 year old filly to a friend. When she was born she was either a darker dun or a lighter bay with a thick dorsal stripe. I can't tell you her foal color for sure, as I only seen her as a foal in pictures. Her dam was a bay snow flake appy and her sire was a black and white tobiano (but I believe he carried sabino). She had 4 tall white socks, a sabino spot in her girth, and snow flakes on her rump. When she shedded out she lost her dorsal and turned into a greyish chocolate color (almost a dark silver color but had a black mane and tail) Well she always looked the same the entire time I owned her but now I just heard from her owner that she has developed a dorsal again, only this time its white. She will try and get a picture of it to me, I will post it when I get it. I have never heard of such a thing. Could be be from the sabino? or the appy? I thought it might be lacing but according to her it's a dorsal. I know the owner very well and she never puts her horses out side in the rain so I know it's not from rain scald. Any ideas what it might be??