It does diagonally mirror (so it mirrors through the center point).
I am not sure what you mean by it mirroring right to left in this case because it seems to be working normal to me. (there also are 2 mirror mines in that level you just havent found one yet that might be where you are having a problem)
this is a 50/50 situation where its either 1,3 from the bottom left or that from the top right. depends which one the last mirror mine is in. you just need to save up something to deal with the 50/50 that will realisticly happen.