Actually, the required fix is in Qt 6.4.2. So the issue is not Qt.
That means the issue is calibre. Looking at changes from calibre 6.14.1 to 6.15.0, my **guess** is the problem change is here: "Comments editor: When copying to clipboard, copy clean HTML rather than the junk Qt produces"
See:
https://github.com/kovidgoyal/calibr...3b85faf8057089
My **guess** is he cleaned up too much ... but ...
Given the above, Sigil/PageEdit will probably not try to workaround this change. calibre users will have to workaround it by using an older PageEdit or some intermediate tool to fixup the html code on the clipboard. It is hard enough to workaround Qt bugs / unwanted changes, I do not want to start doing that for calibre as well.