I think the vertical flip you are experiencing is due to the (0,0) pivot of your UV map being at the bottom left corner rather than the top-left. The darkness of the pixels at the bottom-left here indicate this is where your origin is and is why your image is flipped vertically due to it using OpenGL conventions(Y is Up rather than Y is Down):
OpenGL is the only graphics API that defines its texture-space like this. While DirectX/Vulkan/Metal/After-Effects/etc define the top-left corner of their texture-space as the 0,0-origin.
My unit tests have PixMap and B.Map behaving the same, except for how their scale operation works(B.Map scales about the center (0.5,0.5) while PixMap scales about the origin (0.0, 0.0). In my next update I'll can tweak the texture scaling-operation so that they both scale about the center!