Mirroring a block with arcs

Yesterday I had a strange experience. I had created a template for a cul-de-sac (street configuration with arcs and lines). I made a block of the figure and copied it to a location. Since I needed it flipped to face the other way (it was an offset cul-de-sac) I mirrored it. When I exploded it the further manipulate it, I found that the arcs had been converted to ellipses.The workaroound to this was to mirror the original image and then create the block. When it was exploded it was as the original.