That does not mean "working as intended," that means "You got lucky." It's definitely not a 20% drop rate. Probably closer to a 10% (at least assuming that their random number calculator isn't completely broken). Would be statistically very rare for someone to be able to go through all the maps in the event and get 0-1 drops at 20%-- much more plausible at 10%, and since it's happened to multiple people, that would suggest either a broken / streaky RNG or a lower rate.