tex4ebook: bookmarksetup{startatroot} does not reset bookmark level

I’m compiling a .tex document to .epub with tex4ebook and I would like the back matter of the book (additional chapters, glossary, index, references) to appear in the bookmarks at the top level (same as the front matter and each part of the body of the book). If I’m compiling to .pdf, bookmarksetup{startatroot} resets the bookmark level, but if I’m compiling to .epub, the back matter remains under the last "part". Any recommendations for how I can move the back matter to the top level of the bookmarks are appreciated.

documentclass{book}
usepackage{imakeidx}
usepackage{bookmark}
makeindex[intoc=true]

begin{document}

part{Mammalia}
chapter{Monotremata}
platypusindex{platypus}

backmatter
bookmarksetup{startatroot}

chapter{Afterword}
printindex

end{document}