What is the issue with the HTML Standard?

The last subtest of https://wpt.fyi/results/html/semantics/forms/the-select-element/customizable-select/selectedcontent.html expects that <select>'s value setter does not update the <selectedcontent> element when they are both disconnected, but I don't see anything in the specification that enforces that?

It's not clear that simply adding an "is connected" check here is sufficient as there are other code paths that lead to manipulation.