Quite a coincidence that his food was contaminated with a drug that is widely used by athletes, particularly top level cyclists, and particularly cyclists wanting to lose weight and increase lean muscle mass. Like a hill climber for example.
It’s a drug that is cycled as it becomes tolerated very quickly, so my money is on there having been residual traces left in his body after a pre-tour course. I think that would be more likely than a poor masking agent – there wouldn’t be much need to mask a drug that has gone after 36hrs.
I suspect he took a calculated risk that he wouldn’t get tested until after a stage win, but someone got their sums wrong.