Those washers press onto the bolt pretty tightly and can easily be overlooked....chances are there was one there and you just didn't notice it. Change it again in 1K or so and see how the magnet looks.
I just replaced my engine oil drain plug with the rear drain plug with magnet. Not sure why they don't come that way from the factory....they should.
i'm thinking of the same down the road. u happy with it so far? it fit well right? only reason i could see is the magnet sticks up too far and might hit something?