JQL Query Help: Counting Distinct Users by Challenge ID

I’m trying to run a JQL query to calculate the number of users who joined each challenge — specifically, I want to count the number of distinct users (distinct_id) who fired the JOIN_CHALLENGE event, grouped by the event’s challenge_id property. What I've verified so far:

The issue: Whenever I try to aggregate using .groupBy(['properties.challenge_id', 'distinct_id']) and then count distinct users per challenge, the final result either:

Here's a version of the query I’ve tried:

function main() {
  return Events({
    from_date: '2025-01-01',
    to_date  : '2025-12-31',
    event_selectors: [{ event: 'JOIN_CHALLENGE' }]
  })
  .groupBy(['properties.challenge_id', 'distinct_id'], function(events, key) {
    return null;
  })
  .groupBy(['key[0]'], function(state, row) {
    state = state || { users_joined: 0 };
    state.users_joined += 1;
    return state;
  })
  .map(function(key, value) {
    return {
      challenge_id: key[0],
      users_joined: value ? value.users_joined : 0
    };
  });
}

Despite challenge_id clearly existing in the raw event data, the result still returns:

json

[
  { "users_joined": 0 }
]

What I need help with:

Any clarification or recommended fix would be appreciated. Let me know if you need screenshots or JSON samples.