I dont know why it doesnt work, but for sure the above suggestion cant be right..
One possible reason is that that cubic env mapping , at least doen in this way with the auto gen cood camrea space reflection vector, doesnt work right on flat areas.. so maybe the side of the coin, being flat, doesnt reflect in the expected way.
Also, did you make sure to make the texture a cubemap in script? use bmap_to_cubemap(yourbmap);