Troubleshooting suggestion -
Run the engine on the bad mag until it is warm, shut down and carefully check the cylinder temps - the duff one should be considerably cooler than the others. Swap the plug from that side mag to a good cylinder (make sure you get the correct plug or swap them both to be sure) and repeat. If the cool cylinder has changed you know it is a plug/cylinder related problem, if not you know it is further back.
If you have a mag drop you cannot clear and taxi in on the bad mag it gives the engineers a good clue where to start due to the cool pot.