Thanks for confirming I'm not the only one. I was looking at the bug list for KDE 13 and a similar problem was reported. I am very surprised it is still in KDE 14.
A suggested fix in KDE 13 was to copy the repository setup for Ubuntu from the live install media.
I have subsequently tried copying the *.list files under /etc/apt/sources.list.d from the live iso to the installed KDE 14. This seems to work wonders as long as you remember to unmount the installed KDE 14 disk before rebooting and testing.
I am now in the process of attempting an update. If it works I will consider it "fixed".
[I did the above and it works so I'm calling it a "fix"]