Unable to detect action from multi-user-select using Slack bolt

Below is a simple node slack app that displays the built in multi user select block element to a user. I want to handle the action when the user clicks the submit button in the input so I've set the action_id in the block and added an action listener as per the examples in the docs.

However, the action is not getting triggered; ngrok is showing the incoming /slack/actions request but returning a 404.

What am I missing here?

const { App } = require('@slack/bolt');

const app = new App({
  signingSecret: process.env.SLACK_SIGNING_SECRET,
  token: process.env.SLACK_BOT_TOKEN,
});

app.event('app_home_opened', ({ event, say }) => {
  say(`Hi <@${event.user}>!`);

  const blocks = [{
    "type": "section",
    "text": {
      "type": "mrkdwn",
      "text": "Pick one or more items from the list"
    },
    "accessory": {
      "type": "multi_users_select",
      "action_id": "test_action",
      "placeholder": {
        "type": "plain_text",
        "text": "Select items"
      },
    }
  }];

  say({ blocks });
});

app.action('test_action', async(req, res) => {
  console.log(req);
  console.log(res);
});

app.error(error => {
  console.error(error);
});

(async () => {
  await app.start(process.env.PORT || 3000);
})();

So I found the solution to this by looking at the receiver source code.

As far as I can tell this isn't documented anywhere, but if you want separate urls for events and actions you must specify them yourself:

const app = new App({
  signingSecret: process.env.SLACK_SIGNING_SECRET,
  token: process.env.SLACK_BOT_TOKEN,
  endpoints: {
    events: '/slack/events',
    actions: '/slack/actions'
  },
});