Quoted from Scribbles:
I did read about that, and I'm going to put that to the test. I'll be putting them in a RBION, and a GoTG along with my JP when it arrives. Apparently there is some debate as to which games actually cause the magnetism and how quickly it happens if at all. No better way to find out than throw some balls in there and get flipping I suppose.
It seems pretty random, can happen very quickly or sometimes takes a while. It does say right on the website when you order them though, do not use them in games with magnets.