I would think that you have a slight light leak in the foam light seal at the bottom of the Yashica. When you wound the part used film through the Yashica, you will have done so very quickly, I suspect, so the fogging by the light leak will have been too small to notice. When actually taking pictures with the Yashica, you will have been much slower and each frame will have stayed behind the lens for a lot longer, giving the slight light leak time to do the fogging.