Mario asked me to take over so that you don't have to wait too long. The problem is that some of your Mesh Attachments contain flipped-over triangles, which can be seen here at the shoulder joint (marked in green):
The problem is that the SpineSkeletonRenderer Component needs to generate a single normal for a whole MeshAttachment based on the first triangle orientation of it (the normal needs to face either forward or backward depending on the triangle winding order). Now you have run into the unfortunate case that the first triangle happens to be located at the flipped-over armpit triangle and looks into the opposite side of all other triangles.
The solution would therefore be to either assign vertex weights so that such a flipover to the opposite side does not happen (i.e. by increasing the
RightUpperArm bone influence weight at the armpit vertices, until the outer mesh edges do not cross over any more), or to change the mesh so that a different triangle (which will not flip over at any bone rotation) is the first one.