DITA: subtitles not shown for appendices in TOC and bookmarks list

Using FM2020 to publish a PDF from a bookmap, the appendix chapters appear in the TOC as well as in the list of bookmarks BUT the subtitles are ignored. That is in the main chapters of a document, I beautifully get three levels of bookmarks but in the appendix chapters only the main title is listed for both the TOC and the bookmarks. Does anyone know how to fix this? I would like to have the same structure for the appendix as I have for the main chapters.