I have come across a behaviour change for bookmark creation in FrameMaker 2020 where the first component is not structured. In 2019 the bookmark generation always worked as expected, whether the first component was structured or not. This is also the case for 2022.
The books typically have a structured first component, which has no content used for bookmark generation, followed by a generated table of contents, followed by one or two structured components, which do have content used for bookmark generation and of course for the toc. The toc provides a single paragraph style 'Content' which becomes the root bookmark, with the selected elements (or context labels) supplying the children and sibling bookmarks. This generates bookmarks exactly as expected. However in 2020, if the first component is excluded (I only just found I could do this, rather than deleting the component, while troubleshooting this bug! 😅), the only bookmark generated is 'Content'. In both 2019 and 2022 the children and sibling bookmarks are generated.
The following screen shot shows the expected bookmark generation where the first component is structured and then excluded making the first component the unstructured toc.
And now where the bookmarks are not as expected.
I used the StructFAQ sample (I was surprised to find the book needed a lot of work to get it valid!) to create these examples and to log a bug report (FRMAKER-12521 | Tracker (adobe.com)). I do not expect that this will get fixed as it works in 2022, however this post might save someone else time!
I have attached three StructFAQ zip files for the FrameMaker versions I tested the bug with to the bug report.