Got the magnet working.
When attempting to fasten the diode I noticed that I may have inserted the ground cable in a non-correct way (i.e tightening the screws around the plastic exterior instead of the copper directly), basically creating a current loop causing all the 48V to pass through the little tiny resistor instead of through the much beefier MOSFET. I say 'may' because I'm not sure how else the magnet would work, but a theory could be that the ground cable just barely made contact causing the resistor to become the least cost path.
I've added a diode now for safety as well, and hopefully the magnet will continue to work in production. In any case, I am not touching this now.
Another thought springs to mind - I haven't extensively tested the powerboards with 48V.
I've just noticed that they in fact work and that there has been zero problems shooting around for a while. But how will they stand running game after game?