break-after: avoid on headings not preventing widowed headers at page bottom

When exporting to paginated PDF, using a custom CSS, headings that fall near the bottom of a page are not kept with the following content, resulting in orphaned/widowed headings — a heading appears as the last line on a page with its body text starting on the next page. This was observed across all heading levels.

Here is an example CSS snippet:

  h1, h2, h3, h4, h5, h6 {
    page-break-after: avoid;
    break-after: avoid;
    page-break-inside: avoid;
    break-inside: avoid;
  }

  h1 + *, h2 + *, h3 + *, h4 + *, h5 + *, h6 + * {
    page-break-before: avoid;
    break-before: avoid;
  }

Expected behavior is that a heading near the bottom of a page should move to the top of the next page, keeping it attached to the first element of its content.

Is this a limitation of the rendering engine, or are there other ways to get the desired behavior via CSS?