enqueued pull request type not working as expected (pre-merge hook)

[polyphilz]

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I have a dummy GitHub Action:

name: CI

on:
  pull_request:
    types: [enqueued]
    branches: [main]

jobs:
  build:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v4

      - run: echo "i am a pr"
        if: github.event_name == 'pull_request'

      - run: echo "i am a merge group"
        if: github.event_name == 'merge_group'

      - run: sleep 20

      - run: exit 1

...that I'm using to test out merge queues.

The intended behavior I'm trying to get is that for a given PR, when a user adds it to the merge queue, only then does this specific action run. The reason for that is because the action I'm trying to run will be resource-intensive and I don't want it to run on every commit to a PR; just right before merge. If this action fails, then the merge should be blocked.

However, what's actually happening is that the PR successfully gets merged while the sleep 20 is still running.

Any ideas on what I'm doing wrong here?