Here is the pull request:

<div class="branches">
  <code>devbotsxyz:main</code> ← <code>devbotsxyz:st3fan-patch-4</code>
</div>

<div class="github-info">
  <div class="date">
    opened <span class="discourse-local-date" data-format="ll" data-date="2020-10-13" data-time="03:43:51" data-timezone="UTC">03:43AM - 13 Oct 20 UTC</span>
  </div>

  <div class="user">
    <a href="https://github.com/st3fan" target="_blank" rel="noopener nofollow ugc">
      <img alt="st3fan" src="https://user-images.githubusercontent.com/28052/181091098-d15ddde0-b884-495d-96ef-7b8f5967b0ad.jpeg" class="onebox-avatar-inline" width="20" height="20">
      st3fan
    </a>
  </div>

  <div class="lines" title="1 commits changed 1 files with 2 additions and 0 deletions">
    <a href="https://github.com/devbotsxyz/example-ios-hello/pull/4/files" target="_blank" rel="noopener nofollow ugc">
      <span class="added">+2</span>
      <span class="removed">-0</span>
    </a>
  </div>
</div>

And this is a regular commit to main:

devbotsxyz/example-ios-hello

Example iOS Hello. Contribute to devbotsxyz/example-ios-hello development by creating an account on GitHub.

The first has no annotations and no sign of the check. The latter works as I would expect.

@st3fan,

In the docs about the Checks API, I noticed the following description:

Note: The Checks API only looks for pushes in the repository where the check suite or check run were created. Pushes to a branch in a forked repository are not detected and return an empty pull_requests array.

When the workflow runs on pull request, the commit SHA (github.sha) is the last merge commit on the PR merge branch (refs/pull/:prNumber/merge). It is not a commit that has been pushed into the repository.
At this time, you should specify the latest commit on the head branch (source branch) of the PR as the head_sha of the API. To get the head SHA, you can use the expression ${{ github.event.pull_request.head.sha }}.