Thank you. That is exactly what I needed. The "broken" one has html code without the html and body tags that are required on Windows.
The "working" one clearly has the html and body tags needed on Windows. See the comments in this Qt bug report:
https://bugreports.qt.io/browse/QTBUG-100806
So the problem is actually Window's specific and is caused by Qt changes in qclipboard code in the qtwebengine codebase in Qt 6.4 that should be fixed in Qt 6.5.
The only workaround, I can think of is when the pop-up comes up and the user responds no to pasting plain text, we can use a quick regular expression to check for html and body tags in the html on the clipboard and create a new "fixed" clipboard entry overwriting the current clipboard so that things actually work for Window's users.
That said, I think the entire problem will just go away when Kovid/Calibre moves to Qt 6.5.3.
But I have no idea if or when that might happen.