Writer export to epub: no hyperlinks, bookmarks, headings

The hyperlinks, bookmarks and headings in my .odt still work after export to pdf, but not after export to epub when viewed by Thorium reader. My first experience of any kind with .epub. I have not tried another viewer yet. Does this seem like an export problem, epub limitation or Thorium problem? Am I expecting too much?

Works for me with Aquile Reader (MS store)

Win 11, ver 25.2.5.2

I exported the introduction of the Impress Guide to epub and opened it in:

  • Calibre E-book Viewer. Hyperlinks worked but there were no bookmarks visible and the TOC was not visible
    ExportToEpub_CalibreE-bookViewer

  • Sumatra PDF Reader. Hyperlinks worked, Heading 1 links were visible in a Bookmarks side panel and they worked to jump to different headings. TOC was not visible

Version: 25.8.0.3 (X86_64)
Build ID: ab0112984cb0c13cf3d4edd063593d096a42a1aa
CPU threads: 8; OS: Windows 11 X86_64 (build 26100); UI render: Skia/Raster; VCL: win
Locale: en-NZ (en_NZ); UI: en-GB
Calc: CL threaded