Page 184 - Lucky Coin "Whoever toys with such energies is at risk of suffering from the opposite effect; if the result of the roll is a natural 20, the carrier will have disadvantage on every ability check until they can take a short (or longer) rest." N20s are good in 5e so this should probably be N1.
Natural 1 is nigh impossible as the owner has advantage on the check. Odds are 1 to 400.
Obviously the game designer wanted a drawback, and couldn't choose natural 1. But in the end it feels clumsy as, as you said, natural 20 are positive in 5e, and also, with advantage, the odds are 1 to 10.
The condition could be "if the roll is failed with a natural 1 on any of the dice", which corresponds more to 5e.