Hi All,
I had this problem, with a mesh that wasn't mine. Turned out that some of the faces/vertices were doubled up so that they would face in both directions and i guess SL just gets rid of one on upload, not always the correct one.
Solved it by merging vertices by distance and then flipping any faces one by one that were facing the wrong way recalculate normals outside after that.
Nevermind, that didn't actually work, but i'm convinced it was part of the problem