Overview:
This situation can occur e.g. if ARIA label text is downloaded from server for chosen site/web app localization and added to markdown some time after page load.

Steps to Reproduce:
1). Please use the following code snippet:
<p>Text content</p>
<section aria-label="First section label">
  <h2>First section heading</h2>
  <p>First section content</p>
</section>
<section id="second">
  <h2>Second section heading</h2>
  <p>Second section content</p>
</section>
<script>
  const section = document.getElementById('second');
  setTimeout(() => {
    section.setAttribute('aria-label', 'Second section label');
  }, 500);
</script>

2). Open rotor menu by using VO + U key combination, navigate to Landmarks section

Actual Results: Region landmark is present only for first section

Expected Results: Region landmarks are present for both sections

Build Date & Hardware: Safari 15.5 (17613.2.7.1.8) on MacOS Monterey 12.4

Additional Information:
Although second section landmark is not added to rotor landmarks menu, it is properly read while traversing the page as region with label.