Resolved, but I've done it by removing our use of aria-expanded. Doing this correctly is too hard right now as the implementation has to be much more complex than originally assumed when this was implemented. Additionally we may have a problem with there being an assumption that any expandable section only has a single button, yet we may have a clickable icon as well as a clickable piece of header text.
I have done a big review of ARIA, implemented some simple parts, and am putting together a new issue that documents the more complex stuff that we could do.