This bug is the result (side-effect) of an intentional change in behavior to fix
RT-5534. In the past 2D shapes with 3D transforms are rendered by tessellating the shapes. However when the tessellator was retired, we render this 3D transformed shape onto a texture with existing 2D AA shader and then 3D transform the texture. The dirty edges is the result of the blending an AA mask onto a RTT with depth buffer on. Attached is an image showing the dirty edges on overlapped 2D shapes.